Toward a real time stand-off submillimeter-wave imaging system with
large field of view: quasi-optical system design considerations
Erio Gandini*a, Nuria Llombarta
Terahertz sensing group, Delft University of Technology, Delft, the Netherlands
a
ABSTRACT
In the frame-work of the European project CONSORTIS, a stand-off system for concealed object detections working at
submillimeter-wave frequencies is being developed. The system is required to perform real-time image acquisition over
a large field of view at a short range using both an active and a passive sensor operating in the frequency range from 250
to 600 GHz. In this contribution, the main trade-offs associated with the quasi-optical system design are presented. The
imaging distance is from 2 m to 5 m range with a spatial resolution lower than 2 cm. Focal plane arrays will be used to
achieve high imaging frame rates. Two configurations are considered in CONSORTIS: a sparse array of active
transceivers and incoherent passive staring array with a large number of elements. Both cases use mechanical scanning
to achieve the required field of view.
This paper presents an in-depth analysis of the different trade-offs driving the quasi-optical design: from the mechanical
scanner considerations to the optical beam quality required over the whole field of view. This analysis starts from the
fundamental limitations of the quasi-optical mechanical systems. The limitations of the optics are discussed considering
a canonical elliptical reflector as a reference. After this fundamental analysis, we compare the performances of several
practical standard implementations, based on dual-reflectors and lenses, with canonical geometries. It is shown that, at
short ranges, the main limitation of the optical system is the poor beam quality associated with the wide angular field of
view and none of the standard implementation fulfills the requirements. In the last section, a technique to overcome this
limitation is investigated. In particular, the use of optics with oversized reflectors can significantly improve the
performance over a larger field of view if the coma aberrations are limited by a good angular filter.
Keywords: Imaging systems, submillimeter wave, terahertz, focal plane array, mechanical scanning.
1. INTRODUCTION
Standoff security imagers at sub-millimeter wave frequencies were the subject of intense research in the past [1]-[5]. In
this context, the imagers are divided in two main categories: active and passive systems. The active imagers are based on
a radar configuration, with synthetic or optical imaging schemes, whereas passive ones operate as cameras. The main
challenge in all these systems is to achieve a large field of view (FOV) with satisfactory spatial resolution and fast image
acquisition.
When optical systems are employed for image formation, the use of multiple detectors in the focal plane allows the
image acquisition to take place in parallel with as many pixels as detectors increasing the imaging speed dramatically
when compared to single pixel systems. The main bottleneck in this scenario is the lack of sensitive detectors in large
array formats at submillimeter wavelengths. For passive imaging, the biggest arrays with sufficient sensitivity are
currently based on cryogenic bolometers [6], [7]. These technologies allow the development of sufficiently large linear
arrays that it is possible to image the FOV in one direction. The image is then formed by using a fast scanning in the
orthogonal dimension. This scanner needs to be moving at the same speed as the image acquisition.
The array technology needed for active imagers is less mature than for passive imaging. Therefore, the development of
these imagers will have only 5 to 15 array elements in the mid-term due to the relatively high cost associated to the
submillimeter-wave heterodyne or homodyne transceivers [5], [8]. This implies that the image formation needs a
scanning mechanism in two dimensions, with one scan operating at a significantly larger speed than in the passive
imaging case. Therefore the mechanical challenges of an active scanner are much larger and can, in practice be a
limitation.
Passive and Active Millimeter-Wave Imaging XVIII, edited by David A. Wikner,
Arttu R. Luukanen, Proc. of SPIE Vol. 9462, 946205 · © 2015 SPIE
CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2177038
Proc. of SPIE Vol. 9462 946205-1
Independently of the imager type (passive or active), the main drivers of the opto-mechanical system to be faced are
intrinsically different depending on the chosen imaging distance and frequency. The main differences between short and
large imaging distance is set by the requirements of the mechanical scanner (i.e. physical dimension and achievable
angular speed), and its scanning scheme, that differs in the active (2D) and passive (1D) cases. For the systems
operating at a relative large range (>5m), the angular field of view (FOVθ) to cover a person with a height of 2 m is
relatively small. However, large apertures are required to achieve satisfactory resolution. For example, a 1 m reflector
was used in [5] to acquire images at 25 m. This implies that a large optical magnification (M) is necessary to reduce the
physical dimensions of the mechanical scanner. An optical configuration especially suited for this case is a confocal
optical system coupled to a flat reciprocating mirror [9], [10].
In a short range system, as in CONSORTIS, a person of height 2 m corresponds to a much larger angular FOV. Because
of the short range requirement, the diameter of the main reflector is relatively small. This allows placing the scanner
after the main aperture of the system avoiding the introduction of additional beam aberrations. Therefore, for short range
systems, the challenge associated with the optical system is not related to the mechanical scanning scheme. The main
issue is, instead, to illuminate the quasi-optical system with a focal plane array (FPA) maintaining a good beam quality
for off-focus beams across the image space over a large angular FOV.
In CONSORTIS, an active and passive system will be combined to achieve high probability of detection. The two
systems will operate in the frequency range 250 to 600 GHz and images will be taken in the short to medium range (2 to
5 m) with a real-time frame rate. The resolution (Δρ) is smaller than 2 cm for both imagers and a large FOV is required
(2 m). The main difference in the opto-mechanical active and passive systems lies in architecture of the FPA. A sparse
linear focal plane array of waveguide-based active transceivers [8] will be used for the radar and a large incoherent
passive staring array will be implemented for the radiometer [6]. Since both FPA architectures are not fully sampled, a
mechanical scanning scheme (1D for the passive and 2D for the active) will be needed to completely image the FOV.
In this contribution, the fundamental trade-offs leading the design of the active and passive optical systems at short
ranges are discussed in terms of beam quality. Standard implementations of quasi-optical systems are discussed and
compared to an ideal symmetric elliptical reflector. The use of over-size optics in combination with angular filters
(aperture stops or antennas) can enlarge the FOV as described in the last section.
2. PERFORMANCE OF STATE-OF-THE-ART IMAGERS FOR SECURITY APPLICATIONS
In an imager system, the limit of the FOV is determined by an acceptable reduction in the beam quality. For proper
imaging, a well-focused beam (i.e. small beamwidth, low sides lobes) is needed over the whole FOV. A parameter that
quantifies this beam quality is the antenna directivity or amplitude of the field intensity at the target plane. Therefore, the
scan loss (i.e.
=
|
|
, where
is the field at the target plane of a nominal focusing aperture with the same
diameter and edge taper) is used here as the quality parameter to compare the performance of the different optical
systems.
An acceptable scan loss limit has to be determined before the achievable FOV can be defined. For quasi-optical systems,
the scan loss is divided in to two main factors: phase aberrations and spillover. Phase aberrations affect both the signal to
noise ratio (SNR) and the beam quality (beam enlargement and side lobe level increase). Spillover loss causes a loss in
the SNR and changes the taper of the reflector. The taper variation also degrades the beam quality, but, since it only
affects the amplitude of the radiated field, its contribution is less significant than the one related to the phase aberrations.
Passive imagers at submillimeter wave frequencies were demonstrated successfully in [11]-[13]. The characteristics of
these systems are reported in Table 1. High frame rates over a large FOV were achieved thanks to the use of a large
number of receivers. In these systems, the scan loss is due to the off-focus position of the receivers in the FPA and to the
scanner, that in some cases, [12] and [13], is a focusing mirror. The optical performance in terms of scan loss at the edge
of the FOV was not reported in [12] and [13]. In [11], the resolution at the edge of the FOV is doubled. This
enlargement can be translated in a scan loss of the optical system. In particular, by considering an elliptical symmetric
reflector illuminated by a Gaussian beam with -11 dB edge taper, it corresponds to approximately 3.5 dB loss.
Proc. of SPIE Vol. 9462 946205-2
Table 1. Characteristics of state-of-the-art passive imaging systems at sub-millimeter wave frequencies. The reported
resolution is one-way.
Reference
Optical
architecture
Frequency
(GHz)
FPA
elements
Range
(m)
Nominal
resolution
(cm)
FOV
(cm)
Loss at the
edge of the
FOV
Frame
rate
(images
/second)
[11]
Single
spherical offset
mirror
640
64
8
4 at 5 m
400×200
3.5 dB
7
[12]
Symmetric
Cassegrain
(M = 4.2)
350
64
10-20
1-2
100×100
Not
mentioned +
blockage
25
[13]
Reflector based
(not described)
80-100
32
2-20
Not
mentioned
150×70 to
400×300
Not
mentioned
8-10
Table 2. Characteristics of state-of-the-art active imaging systems at sub-millimeter wave frequencies. The reported
resolution is two-way.
Reference
Optical
architecture
Frequency
(GHz)
FPA
elements
Range
(m)
Nominal
resolution
(cm)
FOV
(cm)
Loss at the
edge of the
FOV
Frame
rate
(images
/second)
[14]
Single offset
mirror
1560
1
2.54
1.5
50×50
Not
mentioned
2
[5]
Offset confocal
dual-reflector
(M = 10)
676.5
1
25
1
40×40
No
significant
scan loss
1
[15]
Single offset
reflector
350
1
5
1
150×250
No
significant
scan loss
0.1
[16]
Lens plus
symmetric
Cassegrain
dual-reflector
300
1
0.5-1.5
0.8 at 1 m
20×30
No
significant
scan loss +
blockage
0.1
[16]
Lens plus
symmetric
Cassegrain
dual-reflector
645
1
0.75-1.5
0.4 at 1 m
20×30
No
significant
scan loss +
blockage
0.1
[17]
Offset bifocal
dual-reflector
(M = 5.5)
300
1
8
1.1
80×50
3.5 dB
1
Active imagers have been realized in the frequency range 200 GHz to 1.5 THz [5], [14]-[17]. The characteristics of these
state-of-the-art active imagers at sub-millimeter frequencies is summarized in Table 2. All the systems shown in the
table have a single transceiver in the focus of the optical architecture. Therefore, low frame rate are achieved when
compared to the systems of Table 1. The scan loss in [14] is not mentioned, but it is expected to be significant since the
scanner is placed between the transceiver and the reflector system. In [15] and [16], the scanner is placed after the main
aperture and does not introduce significant aberrations in the scanned beams. However, in [16], the blockage due to the
symmetric configuration of the reflector introduces additional losses. For this reason, oversized reflectors are used to
achieve the required resolution. Dual-reflector solutions based on Gregorian confocal architectures were proposed in [5]
Proc. of SPIE Vol. 9462 946205-3
and [17]. In these cases, the large range causes the employment of large main apertures to achieve satisfactory
resolutions. Therefore, the scanner is placed before the main aperture of the dual-reflector system and a large optic
magnification is used. Thanks to the relatively small angular FOV (1° × 1°), no significant aberrations of the scanned
beams are present in the design of reference [5]. The scan loss in [18] is larger since the range is shorter and the FOV is
larger (i.e. the angular FOV is larger, 5.7° × 3.6°). The measured two-way resolution of this last system varies from 1.1
to 2.3 cm over the FOV, corresponding to approximately 3.5 dB scan loss.
From the analysis of these state-of-the-art systems, it results that it is challenging to achieve a large FOV with high
frame rate without compromising the optical performance of the system. As it is shown in Tables 1 and 2, real time
frame rate over a wide FOV is achieved only by employing an FPA of receivers, as it is the case of the mentioned
passive systems. However, this in general implies large scan loss at the edge of the FOV, [11]. The performance
degradation at the edge of the FOV brings to a reduced image quality moving away from the center of the target. This
means that an effort has to be made in order to understand how the scanning performance of standard focusing systems
can be improved in order to achieve approximately the same image quality over the entire FOV. As a requirement, a 2
dB scan loss is considered in the following as the limit of the acceptable FOV. By doing a simple Gaussian beam
analysis, this translates into a resolution deterioration of a factor 1.35 (i.e. the resolution is 1.35×Δρref, where Δρref is the
). Note that, for the short range considered in the following (2
reference resolution of the diffraction limited beam,
to 5 m), none of the reported systems can achieve this low scan loss over a FOV larger than 50×50 with real-time frame
rate [14]-[16].
3. CANONICAL OPTO-MECHANICAL SYSTEM
In this section, we investigate the achievable FOV of a canonical opto-mechanical system operating at short ranges: a
symmetrical elliptical reflector fed by a focal plane array coupled to an external mechanical scanner, see Fig.1(a). The
FOV limit established here is used as a reference in the following sections to compare the performance of standard dualreflector and lens architectures.
The basic architecture of the canonical optical system is considered to be the same for the active and passive imagers.
Only short range options (i.e. scanner after the main optics) are investigated. The practical limit of the range will depend
on the mechanical implementation of the scanner.
D,
Elliptical
reflecto
Elliptical
,- reflector
Scanner
R,
F,
Hr"
,
FOV
atitiatineMSinati
FPA
R
PA
FOV
FO
(a)
(b)
Figure 1 – Schematic representation of ideal considered architectures: (a) elliptical reflector followed by a scanner, (b)
symmetric elliptical reflector used for the computation of the scan loss.
Proc. of SPIE Vol. 9462 946205-4
Scanning performance dependence on the reflector f number
A symmetric elliptical reflector, Fig. 1(b), is considered here for the analysis. The focusing distance is Rf = 3.5 m, in the
middle of the proposed range 2 to 5 m. The scan loss was computed at two example frequencies in the range of interest,
300 and 600 GHz. The diameter of the reflector is calculated to have a 1 cm resolution and it is different for the active
and passive systems. Indeed, the resolution for passive systems corresponds to the one-way resolution (Δρ), whereas for
active imagers the two-way resolution is considered (Δρ2way = Δρ/√2).
The main parameter that defines the FOV, once the reflector diameter and frequency are fixed, is the optical f-number
(i.e. the ratio between the focal distance (Fr) and diameter (Dr)). The scan loss typically reduces by increasing the fnumber [18]-[20], however the FPA physical dimension also increases with the f number. Therefore, the scan loss only
reduces if the array elements are tilted towards the reflector centre. This operation reduces the spillover loss, that would
otherwise be significant when the FPA size is comparable to the reflector diameter.
For a 2 dB scan loss the linear and angular FOV are plotted in Fig. 2(a) for the described cases. The feeds are tilted
toward the centre of the reflector and the reflector taper illumination is -11 dB. The scan loss is then mostly due to phase
aberrations since the spillover is negligible. Note that the FOV in Fig. 2(a) represents a two-sided scan (i.e. the reflector
scans ±FOV/2). The curves are obtained by using GRASP [21]. The FOV is larger for the active system. This happens
because, even if the reflector diameter is smaller, the one-way beamwidth is larger and then the same scan for active and
passive corresponds to less beams than in the active case. Since the angular resolution (λ/Dr) and the system
magnification (Rf/Fr) do not change with the frequency, the FOV does not significantly depend on the frequency. The
FOV increases monotonically as a function of the f number. However, the choice of the f number between 2 and 3 is
usually imposed by practical limitations. This allows the system to be kept compact. Moreover, a very large f number
requires very directive feeds that can be difficult to implement.
The radiation patterns normalized to the on-focus beam are shown in Fig. 2(b) for a passive one-way focusing system
with f# = 2.5, working at f0 = 300 GHz. The scan loss of 2 dB is obtained in this case for a FOV of ±45 cm. The beam
quality is acceptable at the limit of the FOV: the resolution is 1.35 times the nominal one and the side lobe level is very
low. This confirms that good quality image can be achieved at the limit of the FOV if the 2 dB scan loss criteria is used.
._ÿ
'
Tw -way
160
2.120
O
18
-
12
One-way
40
m -4
24
...
J
80
o
30
200
p
c
cm
1
v
09 -8
O
6
-fo = 300 GHz
- fo = 600 GHz
0
1
1.5
2
3
3,5
4
0
-20
-10
(a)
0
10
30
20
Scan Km)
40
50
60
(b)
Figure 2 – FOV as a function of the reflector f number. (a) Linear and angular FOV. The diameters of the reflectors (to have
1 cm resolution) are: i) one-way: Dr = 40.2 cm at 300 GHz and Dr = 20.1 cm at 600 GHz; ii) two-way: : Dr = 28.4 cm at 300
GHz and Dr = 14.2 cm at 600 GHz. (b) Scan radiation patterns for the one-way case with f0 = 300 GHz, and f# = 2.5.
Scan performance dependence on the beamwidth
The same analysis as in the previous section was carried out by varying the resolution and the focusing distance. The
chosen f number is f# = 2.5 for practical reasons. The scan loss was computed at two frequencies, 300 and 600 GHz. The
first analysed parameter is the imaging range. In this case, the resolution was fixed to 1 cm and the diameter of the
elliptical reflector was then calculated for every focusing distance in all cases (active/passive, f0). The linear and angular
Proc. of SPIE Vol. 9462 946205-5
FOV to have 2 dB scan loss are plotted in Fig. 3 for the described cases. The linear FOV enlarges as the focusing
distance increases, whereas the angular FOV reduces with the focusing distance. The increase of the focusing distance
makes it possible to scan a larger number of beams as shown in Fig. 3(a) (note that the resolution is 1 cm, therefore, the
ordinate axis in Fig. 3(a) also corresponds to the number of scanned beams). Since the focusing distance is much larger
Δ /
than the cross-range resolution, the angular resolution reduces almost linearly as the range increases, Δ =
≈ Δ / . Here, the FOV does not increase linearly with the focusing range. The two variables have a quadratic
relation, FOV
=
, where c is a constant that depends on the frequency, the reflector f number and the resolution.
This explains the decrease of the angular FOV as the range increases, indeed, the angular beamwidth reduces faster than
the number of scanning beams, resulting in a decrease of the angular FOV.
200
0£
SZ
-f0
= 300 GHz
---fo = 600 GHz
40
2.5
3.5
3
4.5
4
00£ zHD
°J- _ 009
zHD
=
SZ
5
£
S'£
S'b
b
S
ImPx
R,-(m)
(a)
(b)
Figure 3 – FOV as a function of the focusing distance: (a) linear FOV, (b) angular FOV.
If the focusing distance is fixed, the diameter of the reflector depends on the beamwidth (BW) of the scattered field.
Note that the beamwidth corresponds to the resolution in one-way systems, whereas in two-way systems it has to be
divided by √2 to obtain the resolution. In Fig. 4, the linear and angular FOV as a function of the beamwidth are reported.
The focusing distance is Rf = 3.5 m and the f number f# = 2.5. Both the linear and angular FOVs enlarge as the resolution
increases. Therefore, the FOV is limited by the requirement of having a resolution smaller than 2 cm.
250
40
200
ffi-
>
0
w
150
100
50
- fa = 300 GHz
fo = 300 GHz
fo= 600 GHz
---fa = 600 GHz
005
1
15
25
2
Beamwidth (cm)
3
3.5
4
00 5
(a)
1
15
25
2
Heamwidth (cm)
3
3.5
4
(b)
Figure 4 – FOV as a function of the width of the scattered beam: (a) linear FOV, (b) angular FOV.
The FOV in the canonical system can also be limited by the scanner rotation. The scan loss introduced by the scanner is
represented in Fig. 1(a). The scan is performed on a sphere of radius R2, whereas the FOV is a plane. The mismatch
Proc. of SPIE Vol. 9462 946205-6
between the two surfaces results
r
in a scaan loss. As the focusing distaance increases,, the radius R2 also increasess and the
o the FOV. Therefore,
T
for shorter
s
ranges this effect is more relevant. As an
scan path shhape approachees the plane of
example, thee shortest rang
ge considered in
i this manusccript was chossen (R2 = 2 m)
m and the lossses evaluated by
b using
GRASP. Thee scan loss for a 2 m FOV is less than 0.5 dB.
d This loss is
i much smalleer than the one introduced byy the offfocus radiatiion of the refllector. Therefoore, it is possible to concluude that the looss associated to the scanneer is not
significant foor the evaluatio
on of the perforrmance of the system
s
and it will
w not be furtther discussed here.
h
4. STANDAR
RD OPTICAL
L SYSTEM IMPLEMEN
I
NTATIONS
d
in the previous secction is not ussed in practicee because the FPA
F
is in fronnt of the
The symmettric reflector described
aperture of thhe reflector and leads to blocckage. Dual-refflector architecctures are usuaally the adoptedd solution for this
t kind
of focusing systems.
s
Threee significant exxamples of such architecturees are reportedd schematicallyy in Fig. 5(a)-(c) [18],
[22]. The firrst one is a sym
mmetric Gregoorian architectuure. The blockkage is reducedd compared to the symmetric ellipse
since only thhe secondary reflector and not
n the entire FPA is in froont of the maiin reflector apperture. The otther two
architectures in Fig.s 5(b) and 5(c) are an
a offset Greggorian and an offset Dragoniian solutions with
w no blockaage. The
M), i.e. the ratioo between the main
m
and sub reflector
architectures of Fig. 5(a)-(c) have differeent optical maggnification (M
t blockage, the
t secondary reflector
r
of thee symmetric Grregorian is choosen to be one fourth
f
of
focal distances. To reduce the
r
The optic magnificcations of the offset Gregoriian and Dragonnian are 2 andd 1.3, respectivvely, and
the primary reflector.
they fulfill thhe Mizugutchi condition [23]. The reflectorr architectures are also compaared with a hypperbolic refracctive lens
[24], shown in
i Fig. 5(d). Th
he lens materiaal considered inn the analysis is
i HPDE (n = 1.5,
1 tanδ = 0.0001).
Secondi
reflect
Feed
(a)
ry reflector
(b)
rimary reflector
(c)
I.
i
(d)
Figure 5 – Schematic rep
presentation of thhe focusing systeem architecturess compared in this section: (a) syymmetric Gregorrian;
(b) offsett Gregorian; (c) offset
o
Dragoniann, (d) dielectric refractive
r
lens.
P [21] for the reflectors
The scanningg performance of the focusinng system archhitectures were simulated by using GRASP
and Zemax for the lens [2
25]. The loss of the lens annalyzed here includes
i
phasee aberrations, transmission
t
loss, and
A the system
ms work at the frequency f0 = 300 GHz, annd have the sam
me f number, f# = 2.5,
dielectric abssorption loss. All
focusing disttance, Rf = 3.5 m, and taper illumination,
i
Te = -11 dB. Thhe diameter of one-way and two-way
t
cases is again
calculated to have 1 cm ressolution. The scan
s
loss is shoown in Fig. 6 and
a compared to
t the one of thhe symmetric elliptical
e
reflector desccribed in the prrevious sectionns.
The symmettric Gregorian starts with a 1dB loss withh respect to thhe reference caase due to thee blockage. Thhe offset
solutions, innstead, have a comparable field level wiith the referennce at the cennter of the FooV, however they
t
are
characterizedd by an asymm
metry in scannning when thee feed displacement is in thhe offset planee. The front-ffed dualreflector Draagonian architeecture (Fig. 5(cc)) shows a muuch larger FOV
V than the twoo other reflectoor solutions. The
T main
reason is thaat, with this sysstem, the opticcal magnificatioon is smaller thhan in the otheer cases. The Dragonian
D
and the lens
have comparrable phase ab
berration loss. However, the transmission and
a absorptionn losses in thee lens reduce the
t FOV
drastically. In
I particular, the most lim
miting factor is the dielecttric absorptionn loss that beecomes signifficant at
submillimeteer wave frequeencies. The cenntral thickness of the lens forr one-way andd two-way casees is 6 cm andd 3.5 cm,
respectively. By using Gau
ussian beam proopagation analyysis [26], the dielectric
d
absorrption loss can be calculated to
t be 2.5
t one-way case,
c
the total loss (considerring also
dB and 1.4 dB for one-waay and two-waay cases, respectively. For the
Proc. of SPIE Vol. 9462 946205-7
transmission loss) for the on-focus beam is already approximately 3 dB, whereas for the two-way solution a few
centimeters only can be scanned before reaching the 2 dB loss limit. The radiation patterns of the Dragonian dualreflector system are shown in Fig. 6(c) for a one-way system at 300 GHz. The patterns symmetric elliptical reflector for
the same configuration are plotted in Fig.2(b). The quality of the radiation patterns of the Dragonian is comparable to the
one of the symmetric ellipse for the same scan loss.
4
3
3
2
2
t
m
0
ná
m
o
o
0.4
Scan (m)
Scan (m)
(a)
(b)
0.6
08
0
1
t07-4
ö0
8
112
16
20
-60
-40
20
0
20
Scan (cm)
40
60
(c)
Figure 6 – Scanning performance of standard focusing system architectures shown in Fig. 5: (a) one-way systems, (b) twoway systems, (c) radiation patterns of the Dragonian solution at 300 GHz for a one-way system.
5. FIELD OF VIEW ENHANCEMENT
The FOV that can be achieved by using standard architectures is not large enough for security applications where a 2 m
FOV is required. The field of view, as discussed in the previous sections, is limited by the phase aberrations of the
optical system. Among the different types of aberrations, two important limiting factors for the system performance are
the field curvature and coma aberrations. In optics, a well know technic to reduce these aberrations is the use of an
aperture stop [24]. The aperture stop principle of operation is shown schematically in Fig. 7(a) for an off-focus receiver.
The stop aperture blocks the rays that would contribute to phase aberrations. In particular, given an optical system, there
is a position of the aperture stop that minimizes the field curvature and eliminates coma aberrations at a certain image
height [24]. In practice, the aperture stop acts as an angular filter that blocks undesired directions of incidence that would
increase the scan loss. Note that the aperture of the focusing system is given by the aperture stop and that the optics have
to be oversized in order to intercept all the useful rays. The same angular filtering behaviour can be achieved without an
aperture stop by using antennas as receivers in the focal plane of the focusing system. This concept is shown
schematically in Fig. 7(b), where an antenna is placed in an off-focus position of a symmetric elliptical reflector. The
antenna radiation pattern intrinsically filters out the rays that fall outside its angular aperture. Also in this case oversized
optics are needed. This can be seen in Fig. 7(b), where the useful rays do not intercept the central part of the reflector.
Proc. of SPIE Vol. 9462 946205-8
The use of an aperture stop or an antenna to filter the undesired rays depends on the FPA architecture. In a fully sampled
FPA, as is the case for the passive part of CONSORTIS, many receivers have to be placed very close together since they
have to image points that are close in the FOV. In this case, the allowed space for the receiver is not large enough to
accommodate an antenna. Therefore, absorbers and an aperture stop have to be used. Instead, in the active system of
CONSORTIS, a sparse array will be implemented to illuminate the optics. The relatively large distance between the
feeds allows the use of antennas and then to the unwanted rays can be rejected without the aperture stop.
Elliptical
reflector
Optical
Object
point
--
Object
point
system&
Image
height 46:"Image
Blocked ray
/Bløcked
Useful
reflector
surface
-
Blocked ray
point
Receiving
Aperture
stop
antenna
Antenna
pattern
(a)
(b)
Figure 7 – Aperture stop schematic representation: (a) basic concept, (b) aperture stop realized by using an antenna.
The scan loss obtained by applying the two angular filtering concepts are shown in Fig. 8 and 9 for a symmetric elliptical
reflector at 300 GHz. Fig. 8 shows the results for a one-way system with uniform reflector illumination and an aperture
stop as an angular filter. The reflector diameter is 1.5 times the needed diameter to achieve 1 cm resolution at Rf = 3.5 m.
The reference architecture performance is also shown as a reference. The use of the aperture stop permits asignificant
enlargement of the FOV. The 2 dB scan loss is reached for approximately ±63 cm scan, whereas in the nominal case, the
FOV is approximately ±47 cm. This corresponds to an increase of 35% of the FOV. In Fig. 8(a), the case of oversized
reflector (with diameter 1.5Dr) with no stop aperture is also shown. The latter has a larger effective aperture than the
nominal case and for this reason, its on-focus gain is larger. However, since the field curvature and coma aberrations are
not minimized in this case, the gain decreases quickly while scanning. The 2 dB loss FOV is increased with respect of
the nominal architecture, but it is smaller than in the case of oversized reflector with aperture stop. The radiation patterns
of the three configurations are shown in Fig. 8(b) for 2 dB scan loss. In the oversized reflector without aperture stop
case, the uncorrected coma appears clearly in the main lobe. Whereas the beam quality of the reflector with the aperture
stop is acceptable and comparable to the nominal architecture.
4
.....
- Nominal
Oversize,
Oversize, no aperture stop
.
2
o
-Reference
g -5
aperture stop
Oversize,
no aperture stop
\.
-4
.
20
40
60
80
100
-20
-20
0
Scan (cm
20
40
60
80
100
Scan (cm)
(a)
(b)
Figure 8 – Performance of the described oversized reflectors for the one-way system: (a) normalized gain as a function of
the scanning, (b) radiation patterns for 2 dB scan loss.
Proc. of SPIE Vol. 9462 946205-9
The same comparison is shown in Fig. 9 for a two-way system. The taper illumination of the nominal architecture is -11
dB. The angular filtering is realized here by means of antennas as described before. As a result, the enlarged reflector is
illuminated with a reduced taper. The third curve shown is relative to an enlarged reflector, but with the same
illumination as the reference architecture. The gain of the two oversized cases is larger than the reference one. In the
angular filtering case, the gain increase is lower because of the reduced edge taper. As in the previous case, the use of an
oversized reflector permits the achievement of a larger FOV than in the reference case. Also, the angular filtering of the
antenna allows an efficient coverage of a larger FOV than with a fully illuminated oversized reflector. The FOV increase
is, as in the one-way case, 35% by using an enlarged reflector with antenna-based angular filtering. The radiation
patterns of the three architectures are plotted in Fig. 9(b). The beam quality is acceptable for 2 dB scan loss when using
the antenna filtering solution, whereas the uncorrected coma enlarges the beam of the fully illuminated oversized
reflector.
4
o
-Reference
-Nominal
-- Oversize, angular filtering
4
2
n
-Oversize, angular filtering
fi
'--'OVersiu, fully illuminated
...
E
'.
n
\
'
-4
-20
20
40
60
Scan (cm
80
100
;
0
0
(a)
20
40
Scan (cm)
60
80
100
(b)
Figure 9 – Performance of the described oversized reflectors for the two-way system: (a) normalized gain as a function of
the scanning, (b) radiation patterns for 2 dB scan loss.
6. CONCLUSION
In this contribution, the main trade-offs driving the optical design of imagers at submillimeter frequencies were
presented. The considered architectures work in the short to medium range. In this case, the required angular field of
view is very wide compared to long range solutions. Therefore, the main limitation is the scan performance of the quasioptical system employed. A 2 dB scan loss was chosen as the acceptable performance degradation at the edge of the field
of view. The performance of a standard symmetric elliptical reflector was analyzed as a function of the reflector f
number and focusing range. For a fixed resolution, the field of view is almost independent of the frequency since the
needed diameter of the reflector is the same number of wavelength at every frequency. The field of view enlarges
monotonically as the reflector f number and the focusing range increase. Practical implementations of canonical reflector
and lens systems were shown and compared. The architecture that guarantees the largest field of view is the Dragonian
dual-reflector solution. The large field of view compared to the other dual-reflector systems is due to the smaller optic
magnification of the Dragonian. A small magnification leads to reduced aberrations and spillover loss. The Dragonian
also performs better than the standard lens solutions. The main reason is the high dielectric absorption loss of standard
materials at submillimeter wave frequencies. Even if the Dragonian dual-reflector architecture is the one that shows the
best performance among the analyzed cases, it cannot achieve the required FOV for security applications (2 m). In the
last section, the use of an angular filter is proposed to enlarge the FOV. The angular filtering reduces field curvature and
coma aberrations. It can be achieved in two ways: the first one is the use an aperture stop and the second one is the use
of antennas as receiving elements of the FPA. The best configuration depends upon the FPA architecture, the aperture
stop is more suited for fully-sampled absorber-based FPA, whereas antenna filtering can be realized in sparse antenna
Proc. of SPIE Vol. 9462 946205-10
FPA architectures. The angular filtering technique allows a FOV enlargement of 35 %. Additional techniques to enlarge
the FOV such as multiple shaped optical elements will be needed to further increase the FOV.
ACKNOWLEDGMENT
The authors would like to thank the European Commission for the support to this work in the context of the Seventh
Framework Programme (FP7) project Concealed Object Stand-Off Real-Time Imaging for Security (CONSORTIS),
grant agreement no: 312745.
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