LASER INTERFEROMETER GRAVITATIONAL WAVE OBSERVATORY
LIGO Laboratory / LIGO Scientific Collaboration
LIGO
LIGO-T1100467-v2
March 7, 2013
aLIGO Broadband Photodetector
Matthew Evans, Koji Arai
Distribution of this document:
LIGO Scientific Collaboration
This is an internal working note
of the LIGO Laboratory.
California Institute of Technology
LIGO Project – MS 18-34
1200 E. California Blvd.
Pasadena, CA 91125
Phone (626) 395-2129
Fax (626) 304-9834
E-mail: info@ligo.caltech.edu
Massachusetts Institute of Technology
LIGO Project – NW22-295
185 Albany St
Cambridge, MA 02139
Phone (617) 253-4824
Fax (617) 253-7014
E-mail: info@ligo.mit.edu
LIGO Hanford Observatory
P.O. Box 1970
Richland WA 99352
Phone 509-372-8106
Fax 509-372-8137
LIGO Livingston Observatory
P.O. Box 940
Livingston, LA 70754
Phone 225-686-3100
Fax 225-686-7189
http://www.ligo.caltech.edu/
LIGO
LIGO-T1100467-v2
1 Introduction
The aLIGO design call for 2 modulation frequencies (9.1 and 45.5MHz) and demodulation at the
first, second and third multiples of each. The highest of these is 136.5MHz, which is challenging to
meet in the context of existing aLIGO photodetectors (PDs). Furthermore, the aLIGO Arm Length
Stabilization (ALS, M080371) system will require demodulation at 80 and 160MHz, also
challenging in the context of aLIGO PDs.
The objective of this work is to find a solution to the detection of signals from 18MHz to 160MHz
for 1064nm and 532nm optical wavelengths. It should be noted that the beams to be detected will
arrive from the aLIGO interferometer, and thus will not be fixed to the table in position or angle,
which makes the active area of the PD of special concern. Also of note is the fact that these
detectors will not be used for low-noise “Science Mode” signals, and thus can be designed for inair operation and modest noise performance.
2 Search for an existing solution
The first approach investigated was that of simply modifying the existing Length Sensing and
Control tuned RF photodetector design (T1000694) to operate at higher frequencies. While not
impossible, this approach appears difficult since this design is targeted at ultra-low-noise tuned
readout of frequencies below 50MHz.
A survey of commercially available detectors turns up a number of candidates all of which share a
common problem: in order to operate at frequencies above 100MHz, the active area of the PDs is
too small to be used in the context of a suspended interferometer (usually less than ¼ mm2). The
best of these detectors, the NewFocus 1811, was used for some time in iLIGO for demodulation of
the 50MHz SPOB signal, and was eventually abandoned because the small active area resulted in
an unreliable signal. That is, the beam alignment on the table varied enough with time that the
beam would occasionally move off the PD.
It should be mentioned that a Thorlabs PD10CF detector, which claims 150MHz bandwidth, was
purchased for testing. The advantage of the PD10CF, as opposed to the 1811, is the somewhat
larger active area and built-in threads for mounting lenses to the PD case. The ability to mount
lenses to the PD allows us to magnify the PD by a factor of about 4, limited mostly by our lens
mounting precision of about 100 micro-meters, and the ergonomically dictated maximum telescope
length of 3-inchs. (Magnification also implies an angular requirement on the beam impinging on
the image plane of the PD which becomes onerous for magnification factors much greater than 4.)
While the resulting PD image is of respectable dimension (1 mm2 apparent active area), the
PD10CF response was measured to be limited to 100MHz bandwidth, which is insufficient for our
highest frequency signals. A similar approach to the 1811 is likely to fail due to its smaller size
(thus requiring greater magnification), and lack of built-in lens mounting options (thus preventing
precise lens-PD positioning).
2
LIGO
LIGO-T1100467-v2
3 Design
The aLIGO Broadband Photodetector (BBPD) is built around a 2.5mm diameter photodiode
(PerkinElmer/Excelitas FFD-100), coupled to a 50 Ohm RF amplifier (Mini-Circuits MAR-6SM+).
The FFD-100 is a silicon photodiode which offers low capacitance and series resistance with a
modest bias despite its large active area (12pF and 10 Ohm at 15V). The responsivity of this diode
at 1064nm is low, but acceptable at 0.1A/W, while the responsivity at 532nm is good at 0.3A/W.
Using a 50 Ohm RF amplifier to provide the PD’s RF readout allows for a simple design and a
good response over a wide range of frequencies. The trade-offs relative to a resonant detector are
the noise performance of the PD, which is limited by this 50 Ohm transimpedance, and the ability
to remove unwanted frequencies before amplification, although options to add some filtration are
still available. The frontend preamplifier MAR-6SM+ has the lowest noise level (N.F. 2.3dB)
among the monolithic wideband amplifiers in Mini-Circuits’ line-up. The cascade of MAR-6SM+
and GALI-6+ amplifiers provides total 32dB of amplification (i.e. 20dB+12dB), resulting in an
apparent transimpedance (as seen on the output) close to 2 kOhm. This amplifier also has a highpower output with good linearity (1dB compression at 18dBm), and provides amplification from
DC to 4GHz.
The DC path is made to respond up to 100kHz with 2 kOhm transimpedance. The response is noninverting and driven by a standard op-amp (OP27). This choice of transimpedance gives a 10V
output for maximal incident optical power (50mW of 1064nm, or 15mW of 532nm). It also roughly
matches the RF and DC transimpedance, and matches their ranges for a typical modulation depth of
10% (1Vrms of RF for 10V of DC).
Photodiode Resonant Circuit
Low Noise RF Amp
a15
RF Notch
RF High-Pass
COC3a
C3a
GND
PIC3a02 PIC3a01
100n
PIR20 1
COR20
R20
0
GND
PIPD10A
PIL202
COL1
L1
DNS
DNS
COL2
L2
10uH
COTP5
TP5 10uH
COC4
C4
PIL101
PIU102
GND
PIU103
10n
COL21
L21
10uH
GND
PIL2102
PIU104
PIC20a02
COC20a
C20a
100n
PIC20b02
COC20b
C20b
1n
PIC1a01
PIC1a02
COC1a
C1a
DNS
PIC1b02
COC1b
C1b
DNS
PIC2a01
PIC2a02
10n
COC6
C6
10n
PIU202
GALI-6a
GND
Front Panel
COJ2
J2
PIJ201
RF OUT
SMA
PIJ202 PIJ203 PIJ204 PIJ205
GND
DC Amp 0-100kHz
COTP1
TP1
COR11
R11
a15
PIC2b01
PIR1102
COC9
C9
PITP101
PIR1101
PIC2b02
COC2b
C2b
DNS
PIC2302
PIR901
10uH
PIL702
PIC701
PIC702
PIL801
PIL802
10uH
COC7
C7
10n
GND
COR12 COR13
R12
R13
1k 2k
PIR1201 PIR1301
a15 GND
1
PIU301
COC13
C13
1n
DNS
5
COU3
U3
OP27GS
PIU305
6
PIU306
3
PIC1301
PIC1302
8
PIU308
PIU304
PIC2401
PIC2402
GND
PIC1402 PIC1401
a15
PIU307
PIU303
PIR1202 PIR1302
PIR902
2k
PIC2301
100n
2
PIU302
COL8
L8
COC14
C14
COR9
R9
1n
GND
COL7
L7
Compensation Cap
PIC902 PIC901
1k
COC23
C23
PIL701
GND
PIL2 02
COL22
L22
10uH
PIU203 PIC602 PIC601
GND
MAR-6SMa
DC Low-Pass
PIL301
COC2a
C2a
COC5
C5
GND
PIL302
PIL2 01
COU2
U2
10n
10uH
PIL201
PIC1b01
PIR2202
PIC502 PIC501
PIU201
PIL2002
PIC20b01
1n
GND
GND
PIC20a01
COR22
R22
PIR2201
COC22 150
C22
COL3
L3
COL20
L20
PITP501
PIL2001
COU1
U1
PIC2 01
PIC2 02
7
PIR20 2
PIL102
COPD1
PD1
PIPD10CASE
GND
PIC3b02 PIC3b01
PIPD10K
PIL2101
PIC401 PIC402
PIU101
10n
COC3b
C3b
-15
a15
PIR2102
680
COC21
C21
4
Bias Voltage
COR21
R21
PIR2101
PIC2101
PIC2102
High Power RF Amp
-15
COC24
C24
100n
COR10
R10
PIR1001
100
COJ1
J1
PIR1002
PIJ101
PIJ102
DC OUT
BNC
GND
GND
Figure 1: Circuit schematic of aLIGO Broadband photodetector
Mounting Hole
3
LIGO
LIGO-T1100467-v2
COH1
COC33
COC32
PAC3301
PAC3202 COD1 PAD102
PAR3101
COH2
PAD101
PAH101
PAR3002
COR22
PAD104 PAD103
PAJ205 PAJ202
PAJ201
PAJ204 PAJ203
PAC3302
PAL3102
PAL3002
COL31 COL30
PAL3101
COC31 PAC3102 PAC3101
PATP301
PAL3001
COC2 PAC2201PAC2101 PAC2202PAC2102 COC21
COR21
COC20a
PATP201 COTP5 PATP501
COTP4
PATP401
COP1
COH3
COL20
COR20
PAH301
PAL2202
PAJ104
COC20b
COC1a COC1b
PAC1a02 PAC1b02
PAC1a01 PAC1b01
COC34PAC3401
PAC3402
PAJ102 PAJ101
PAU203
COU2
PAU202
PAU103
COU1
PAU101
PAC402
PAC401
PAC3a01 PAC3b01
COPD1
PAC3a02 PAC3b02
COC4
PAL101 PAL102
PAR902 PAC901
PAR901 PAC902
COR9 COC9
COR10
PAL302 PAL301
PAPD10CASE COL2 COC2a COC2b
PAC2a02 PAC2b02
PAL202 PAL201
PAC2a01 PAC2b01
COJ1
COC7
PAC702 PAC701
PAL701 PAL702
PAC2402
PAR1201
COU3
PAR1102 PAR1101
COL7
COC14
PAC1402 PAC1401
COC24
PAC2401
PAU301 PAU302 PAU303 PAU304
COL3 COSN COR11
PAPD10K COC3b
COL1 PAPD10A
PAR1001 PAR1002
PAU308 PAU307 PAU306 PAU305
PAC502 PAC501 COC5
PAU102
COC3a
COC23
PAC2302 PAC2301
PAU201
PAL2102
PAC20a01 PAC20a02
PAC20b01 PAC20b02
PAL2001 PAL2002
PAR2002 PAR2001
PAJ103
PAC601 PAC602
COL22
COL21
PAR2101 PAR2102 PAL2101
COTP2
COC30
PAP103 PAP102 PAP101
PAC3002 PAC3001
PAH201
COC6
PAC3201 PAR3102COR31 PAR3001 COR30 PAR2 01 PAR2 02 PAL2201
COTP3
COJ2
COC13 PAC1301
PAC1302
PAL802
PAL801
COL8
PAR1202
PAR1302
PAR1301
COR12
COR13
PATP101
COTP1COH4
PAC3501 COC35
PAC3502
PAH401
Figure 2: Design of the PCB (left) and the photo of the assembly (right)
4 Response
The RF response of the BBPD is designed to be high in the region of interest to aLIGO
interferometer sensing and control; 18 to 160MHz. The high frequency end of the response is, in
principle, limited by the time constant determined by the junction capacitance of the diode and the
transimpedance of the preamp. This yields the cutoff frequency of 160MHz. The low frequency
photocurrent (f<1MHz) is guided to the audio frequency circuit by the blocking filters (C3s, C4,
and L3) before the RF amplifier. It should be noted that to reach lower RF frequencies it is
sufficient to replace L1 and L2 with larger inductances (e.g., 10uH), and C1, C2 and C4 with larger
capacitances (e.g., 10nF), though the DC path should be modified slightly to compensate (e.g., C9
and C15 set to 1nF). Similarly, these components can be modified to reject frequencies below a
given cut-off (currently 5MHz), or even to notch unwanted frequencies (e.g., 9MHz). Additional
spare pads (L1, C1a, and C1b) are provided to allow further flexibility of internal filtering such as a
notch filter.
Some complication is added to the DC path in order to provide a 25V bias to the PD (R12 and R13
on the schematic). The balanced design rejects noise entering via the +15V supply, but there is
clearly some compromise to the DC path simplicity and noise performance required in trade for RF
response above 50MHz (see figure 4, response vs. bias voltage). Therefore, the PCB is designed to
allow an external high voltage bias at TP5 while the original bias is to be decoupled by removing
R20. Note that the actual bias voltage across the diode is V(TP5) – 10V (i.e. the original bias
voltage is –25V) as the cathode voltage is determined by the virtual short of the DC current
amplifier (TP1).
shows the response of the BBPD. The transimpedance is 2.3 kOhm at 15MHz where the
maximum response is achieved. With the default configuration, the response goes down to 890
Ohm at 160MHz. The high frequency response can be extended up to 200MHz by applying an
external high-voltage reverse bias at TP5, by removing R20 to decouple the original bias voltage.
This capability of extended response was demonstrated by a proof-of-principle setup circuit
depicted in Figure 4.
Figure 3
4
LIGO
LIGO-T1100467-v2
aLIGO Broadband PD response (2011/7/5)
4
Transimpedance (Ohm)
10
3
10
2
10
1
10
Phase [deg]
0
10 5
10
180
120
60
0
−60
−120
−180 5
10
6
7
10
8
10
6
7
10
9
10
10
8
10
9
10
10
Frequency [Hz]
Figure 3: Magnitude and phase response of the aLIGO broadband photodetector
Perkin Elmer FFD−100 / ERA−5SM
3
10
Hamamatsu S3399
or
Perkin Elmer FFD-100
-VBIAS
+15V
1n
RF out
10uH
(RDC=7!)
1n
1n
ERA-5SM
Transimpedance [Ω]
2
150
470pF SMD
(500V)
10
Vbias
3V
5V
10V
20V
25V
30V
50V
100V
−3dB line
1
10
10!
0
10 5
10
6
10
7
10
Frequency [Hz]
8
10
9
10
Figure 4: Response of the photodiode with various bias voltages (right).
This test was performed with a test circuit (left).
5
LIGO
LIGO-T1100467-v2
The spectral response of the BBPD was also measured. For 1064nm light, it was found to be
0.08A/W and 0.3A/W for 633nm light. These correspond to the expected spectral response of the
FFD-100 only if the overall efficiency of the transmission of light into the PD is around 70% (see
figure below, taken from the FFD-100 data sheet).
Figure 5: Spectral response of FFD-100
5 Noise performance
The output noise of the BBPD was measured by changing the power of the incident light from an
incandescent light bulb. The average noise levels between 15MHz and 20MHz were plotted against
the DC photocurrent flowing on the diode (Figure 6). From the dependence of the noise on the DC
photocurrent (iDC), the (average) transimpedance (gdet) and the shot-noise intercept current1 (idet)
can be obtained. The measurement indicates that the BBPD will be shot-noise limited when the DC
photocurrent is more than 0.22mA.
The frequency dependence of the shot noise intercept current is obtained from the dark noise
current (Figure 7). This was calculated from the output noise voltage (i.e. dark noise voltage) by the
transimpedance. The shot noise intercept current at 160MHz is about 3mA. Since the current noise
level is limited by the reduced response of the photodetector at the high frequency, this noise level
can be improved by increasing the bias voltage.
These photocurrents can be converted to input power at some wavelength with the responsivity
values given in the previous section.
1
The shot-noise intercept current is the level of the DC photo current where the contribution of shot noise becomes
comparable to the dark noise of a photodetector.
6
LIGO
LIGO-T1100467-v2
aLIGO Broadband PD shotnoise intercept current: 15MHz~20MHz
−6
10
Vn: Output noise level [V/rtHz]
BBPD
fitting: Vn = gdet sqrt[2e (i dc + idet)]
transimpedance (gdet): 2.33e+03 Ohm
shot noise intercept current (idet): 2.22e−01 mA
measured data
−7
10
−8
10
idet = 0.222 mA
−7
10
−6
10
−5
10
−4
10
−3
−2
10
−1
10
idc: DC photo current [A]
10
Figure 6: Shot noise intercept current measured with illumination from an incandescent light.
aLIGO Broadband PD current noise level
shotnoise
intercept
current
Current Noise [pA/rtHz]
2
10
10mA
1mA
1
10
0.2mA
0.1mA
6
10
7
10
8
10
Frequency [Hz]
Figure 7: Frequency dependence of the input intercept current
7
LIGO
LIGO-T1100467-v2
6 Interfaces
The electrical interface of the BBPD has two outputs and one input, all located on the topside of the
case. The RF and DC outputs are provided by SMA and BNC connectors. These match aLIGO
electronics (c.f., T1000044 and D1002932) and provide the user with a clear indication of the
purpose of each output (augmented by labeling next to each). The input power, a dual-15V supply,
uses an M8-3pos connector. This matches the power provided on aLIGO optical tables (see
D1002932) for commercial detectors by Thorlabs and Newfocus. Green LEDs adjacent to the
power connecter give a visual indication that the BBPD is properly powered.
The mechanical interface to the optical table is provided by an 8-32 threaded hole on each side of
the BBPD case, which mates with standard optical posts. There are 4 additional holes provided on
each side to allow for specialized adapters. None of these holes penetrates the BBPD case, so the
RF shielding is not compromised and there is no risk of damage to the BBPD due to screws
inserted into the case. Furthermore, the BBPD case provides a threaded bushing centered on the
photodiode that matches standard lens tubes, ND filters, etc.
Figure 8: CAD images of the chassis for the BBPD. Backside (left) and Front side (right).
The BBPD case is attached to its electrical ground, so care must be taken to avoid ground loops.
When the BBPD is mounted on an optical post, the post should be electrically isolated from the
table. Electrical breaks of this sort were used in initial LIGO and took the form of a dielectric
pedestal which mates to standard optical posts.
8