Japanese Journal of Applied Physics 48 (2009) 03A039
BRIEF NOTE
Mode-Hopping Detection Technique for External-Cavity Laser Diodes
Masaki Tanaka, Shinya Yoshida, Yukiko Nagasaka, Tetsuo Saeki1 , Yukio Watanabe,
Hiroyuki Oka2 , Takahiro Miyake, Tetsuo Ueyama1 , and Yukio Kurata
Optical Module Technology Research Laboratories, Production Technology Development Group,
Sharp Corporation, 2613-1 Ichinomoto, Tenri, Nara 632-8567, Japan
1
A1257-1 Project Team (A), Sharp Corporation, 247 Sojo, Nutanishi, Mihara, Hiroshima 729-0474, Japan
2
Engineering Department 2, Compound Semiconductor Systems Division, Electronic Components Group,
Sharp Corporation, 247 Sojo, Nutanishi, Mihara, Hiroshima 729-0474, Japan
Received September 16, 2008; revised November 28, 2008; accepted December 5, 2008; published online March 23, 2009
We developed an external-cavity laser diode (ECLD) for a holographic memory system with a wavelength tunability of 5 nm. To stabilize the
single-mode operation, a mode-hopping detection technique using a time differential of output voltages of a single-element photodiode is
proposed. By changing the drive current of the laser diode, we could obtain mode detection signals when the ECLD was in multimode
operation. This result indicates that this technique is suitable for maintaining a stable single-mode operation of the ECLD.
# 2009 The Japan Society of Applied Physics
DOI: 10.1143/JJAP.48.03A039
C
onsiderable studies have been conducted to develop
holographic memory systems and their recording
density has exceeded 500 GB/in.2 .1–3) In addition,
the holographic read-only memory (ROM) reader drives for
consumer electronics are under development.4,5) For such
ROM drives, a small wavelength-tunable light source is
required. In the holographic memory systems, information is
recorded onto the media, in many cases made of photopolymer materials, as three-dimensional interference patterns.6) Generally, holographic media, made of photopolymer materials, are markedly affected by temperature.
Thermal expansion or shrinkage of the material causes
geometric deformation of recorded holograms leading to
defective reconstruction of holograms.7) Such a dimensional
change is compensated for by tuning both the wavelength
and incident angle of the reference beam to match Bragg
diffraction conditions. An external-cavity laser diode
(ECLD) is suitable for use as the light source of the
holographic memory because of various advantages, such as
its wide-range wavelength tunability, pure single-longitudinal-mode operations, cost effectiveness, and compact size
compared with other lasers.8) However, the ECLD has
several cavities formed by the reflection grating and rear
facet of a laser diode, the rear and front facets of the laser
diode, and the front facet and the reflection grating. These
cavities may disturb the single-longitudinal-mode operation
of the ECLD and cause mode hopping between modes. To
avoid mode hopping, a mode-hopping detection method
using a multisegmented photodiode has been used and
stabilizes the single-mode operation.9,10) In this paper, we
propose a mode-hopping detection technique using a time
differential of output voltages of a single-element photodiode, which will allow a cost reduction of the ECLD in
future manufacturing processes and a simple algorithm for
mode detection. We detected rapid fluctuations of output
voltage caused by mode hopping and successfully determined the drive current value to avoid multimode operation.
Figure 1 shows a schematic of the optical setup. A Littrow
arrangement is used to shorten the external cavity and
maintain a stable single-longitudinal-mode operation. The
diffraction grating and mirror are mounted on the same rotor
to maintain the direction of the output beam when the
wavelength is changed. The thermistor and peltier device are
SIDM Unit
Rotor
Single-element photodiode
Tilt sensor
Wedge beam splitter
Variable attenuator
Output power monitor
Grating
Wedge beam splitter
Collimator
(f = 4.02 mm, NA = 0.6)
Laser diode
Fig. 1. (Color online) Schematic of the optical setup.
attached to the laser holder to control the temperature of the
laser stem. The rotor is turned by a smooth impact drive
mechanism (SIDM) unit, which is used in an optical pick-up
module. The resolution of the rotation angle is 0.0001 /
pulse. The absolute wavelength is controlled using the
rotation angle of the rotor measured by the tilt sensor.
The light beam emitted from the blue laser diode is
collimated by the collimation lens ( f ¼ 4:02 mm) and
diffracted by the grating (grating resolution: 3600/mm).
The first-order diffraction efficiency of the grating is
designed to be 20% to balance the stability of the external
cavity mode and the output power. The first-order diffraction
beam passes through the collimation lens again and returns
to the facet of the blue laser diode. The 0th-order diffracted
beam is reflected by the mirror, passes through the wedge
prism, and becomes the output light beam. The light beams
reflected by both surfaces of the wedge prism interfere, and
the fringe patterns are detected by the photodiode. We detect
mode hopping using the time-differential value of the output
voltage of the single-element-photodiode, and we can
assemble the setup without precisely aligning fringe patterns
to the element of the detector. Moreover, the manufacturing
tolerance of the wedge prism can be relatively large.
The ECLD selects a single wavelength from the optical
gain range of the laser diode using the wavelength selectivity
of the grating and external cavity. The oscillation mode
spacing of the external cavity, ECLD , is expressed as
03A039-1
# 2009 The Japan Society of Applied Physics
M. Tanaka et al.
0
35
-5
30
Output power (mW)
Normalized intensity (dB)
Jpn. J. Appl. Phys. 48 (2009) 03A039
-10
-15
-20
-25
-30
-35
407.48 407.50 407.52 407.54 407.56 407.58
ECLD
Laser Diode
25
20
15
10
5
0
Wavelength (nm)
0
Fig. 2. Normalized spectrum of the ECLD.
Fig. 3.
ECLD ¼
2
¼ 6:4 103 nm;
2ðLLD Neff þ LEC Þ
ð1Þ
LD
40
60
80
100
Output power (mW)
ð2Þ
Ideally, the reflectivity of the front facet is desired to be as
low as possible to suppress the disturbing resonators formed
by the front facet of the laser diode, the grating and both
facets of the laser diode.11) The front facet of the laser diode
is coated with a SiO2 antireflection layer whose reflectivity
is 0.25%. The reflectivity of the rear facet is set to be 95%.
As shown in Fig. 2, the ECLD is operated in the single
longitudinal mode with a side-mode suppression rate of
15 dB. This rate may be improved by modifying the
wavelength selectivity of the grating and lowering the
reflectivity of the front facet of the laser diode. The line
width of the ECLD was within 7 103 nm full width at
half-maximum. Two side modes at around 30 103 nm
from the center wavelength were observed, which were
attributed to the neighboring laser diode chip modes.
Figure 3 shows the optical output of the ECLD. The
threshold current of the laser diode was 62 mA and that of
the ECLD was 52 mA. Generally, the higher reflectivity of
the cavity reduces the threshold current, resulting in a
threshold current difference of 10 mA. As shown in this
figure, we could achieve a maximum optical output of up to
30 mW when the current was 95 mA. However, the operation
at this current may reduce the lifetime of the laser diode.
Figure 4 shows the relationship between the wavelength
and optical output of the ECLD at an injection current of
90 mA. The optical output reached a maximum value at a
wavelength of approximately 410.7 nm and then decreased
as the wavelength decreased above the wavelength of
410.7 nm. The optical output profile is asymmetrically
shaped with respect to the wavelength of 410.7 nm. This
asymmetry was considered to be due to the output spectrum
of the laser diode and the coupling efficiency of the light
Optical output of the ECLD and laser diode.
30
where LLD is the cavity length of the laser diode, LEC is the
external cavity length, and Neff is the effective refractive
index of the laser diode. We use a blue laser diode whose is 408 nm, LLD is 800 mm, Neff is 3.4, and LEC is 10.3 mm.
Sometimes, the single-mode operation is disturbed by the
neighboring laser diode chip modes whose spacing LD is
expressed as
2
¼
¼ 30:6 103 nm:
2LLD Neff
20
Injected current (mA)
25
20
15
10
5
0
407
408
409
410
411
412
Wavelength (nm)
Fig. 4.
Relationship between the output power and the wavelength.
beam diffracted by the grating to the laser diode facet
through the collimating lens. The coupling efficiency could
be changed when the wavelength was changed by rotating
the grating because of the assembling accuracy; accordingly,
the oscillating gain of the ECLD was considered to also be
changed. We defined the allowed power variation as 30%
from the maximum, and the tunable range of the wavelength
is 5 nm.
To compensate for Bragg detuning caused by a temperature variation between recording and reading, wavelength
tunability should be an indispensable property. We estimated
how many wavelengths are required when we use the
holographic memory system described in our previous
paper.12) We assume that the numerical aperture of the
objective lens is 0.5, the incident angle of the chief signal
ray is 40 , and the wavelength of the light source is 405 nm.
When a photopolymer material is held between glass
substrates, we can neglect the thermal effect on the
substrates because the expansion coefficient of glass is very
low compared with that of photopolymer materials.7) When
temperatures differ by 10 and 20 C from that of
recording, the required wavelengths were estimated to be
2:4 and 4:8 nm, respectively. Accordingly, our laser
could compensate Bragg detuning caused by the temperature
variation of 10 C.
As described above, wavelength tunability is important
when holograms are read and the stability of the single-mode
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# 2009 The Japan Society of Applied Physics
operation is important when holograms are recorded. A pure
single-mode operation of an ECLD is sometimes disturbed
by drift of laser driving current and ambient temperature,
and migration into multimode operation. When the ECLD
that oscillates in multiple longitudinal modes is used in the
holographic recording system, the contrast of recorded
interference patterns decreases, resulting in a waste of the
dynamic range of recording media.
In this paragraph, we describe how we detected mode
hopping and maintained single-mode operation. The light
beam is reflected by the mirror mounted on the rotor and led
to the wedge prism, the angle of which is 0.05 . The lights
reflected at the incident and exit surfaces of the wedge prism
enter the photodiode and form interference patterns on the
detector of the photodiode. The photodiode has a singleelement detector whose width is designed to be one-half the
cycle of the fringe pattern (124 mm). The sampling frequency
of the photodiode is 500 kHz. When ECLD operates in the
single mode, the signal VPD detected by the photodiode is
almost constant and the time variation of VPD decreases.
When mode hopping occurs, the interference pattern turns
to black and white rapidly, and the time variation of VPD
increases. We define the operation-mode detection signal
SM as
SM ¼
VPD
;
t
ð3Þ
where VPD is the difference between the maximum and
minimum values of VPD . We set the unit time t at 10 ms.
The maximum and minimum values of VPD were selected
from the detected values in t. SM is compared with a
threshold value Sth , and if SM is larger than Sth , the current
state is judged as the multimode operation. By increasing the
drive current, we detect SM and record the drive current
value as In when SM is larger than Sth . After changing the
drive current, a pair of In and Inþ1 which have the largest
value of jInþ1 In j is searched and the intermediate value
ðIn þ Inþ1 Þ=2 is set as the new drive current. By using these
procedures, mode hopping is actively avoided and the
single-longitudinal-mode operation can be maintained.
Figure 5 shows wavelengths and SM obtained when the
drive current was increased from 70 to 102 mA at a rate of
1 mA/s. With increasing drive current, the wavelength
increased. This is because the numbers of electrons and
holes in the active layer of the laser diode increased, with
increasing injection current; accordingly, the effective
refractive index Neff increased, leading to an increase in
the optical length of the external cavity. When the wavelength increased and entered the edge of the range selected
by the grating, the operating mode competed with neighboring modes and mode hopping occurred. As the wavelength
shifted with increasing drive current, fringe patterns moved
gradually on the photodetector, the output signal voltage
varied in a sinusoidal shape, and the time differential of VPD
decreased. When the drive current was tuned to the value of
multimode operation, the fringe patterns fluctuated rapidly
and the deviation value of output voltage increased. Thus,
401.35
1.6
401.30
1.2
401.25
0.8
401.20
0.4
401.15
65
0
70
75
80
85
90
95
Mode detection signal (V)
M. Tanaka et al.
Wavelength (nm)
Jpn. J. Appl. Phys. 48 (2009) 03A039
100 105
Injected current (mA)
Fig. 5. (Color online) Wavelengths and operation mode detection
signal SM of the ECLD.
we could determine the value of the drive current at which
the mode hopping occurred. Mode detection signals were
obtained when the ECLD was operated in multiple modes, as
shown in Fig. 5. This result shows that the proposed
technique is suitable for mode-hopping detection.
In conclusion, we developed an ECLD that operates in the
pure single longitudinal mode and proposed a mode-hopping
detection technique for the ECLD using a time differential of
output voltages of a single-element photodiode. By changing
the drive current of the laser diode, we could obtain a mode
detection signal, which indicates that this technique is
suitable for maintaining the single-mode operation of the
ECLD.
Acknowledgements The authors thank Dr. T. Kira for supporting
our study, and the members of Engineering Department 1, Compound
Semiconductor Systems Division, Electronic Components Group, and
Laboratory 1, and Advanced Technology Research Laboratories, Sharp
Corporation, for providing antireflection-coated laser diodes.
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