Accepted for publication in IEEE PTL March 2004.
Injection-Locked 1.55 µm Tunable VCSEL for
Uncooled WDM Transmitter Applications
Lukas Chrostowski, Chih-Hao Chang, and Connie J. Chang-Hasnain
Abstract— We study the temperature effects on injectionlocked VCSELs for use in WDM transmitters. We show that
injection locking can be used to lock an uncooled VCSEL to the
WDM grid while improving its analog performance.
Index Terms— VCSELs, injection locking, analog modulation.
I.
INTRODUCTION
The
emission wavelength of a semiconductor diode laser
varies with temperature, fundamentally because the bandgap
energies of III-V materials vary with temperature. For lasers
used in wavelength division multiplexed (WDM) systems, the
temperature-dependent wavelength variation can cause
significant crosstalk between neighboring channels. This
necessitates accurate temperature control and wavelength
stabilization to ensure that each laser operates on the WDM
grid. The addition of temperature and wavelength feedback
control leads to high power consumption, large size, and an
increase in packaging cost. WDM transmitters which can
maintain their wavelength without the use of temperature
controllers, known as uncooled WDM transmitters, are thus
extremely attractive for low-cost applications, e.g. fiber to the
home. A continuously wavelength-tunable laser can be used
as an uncooled WDM transmitter using a simple feedback
circuit with a temperature sensor, or alternatively, being
injection-locked by another laser source.
In this paper, we demonstrate a new uncooled WDM
transmitter configuration that uses injection locking to
stabilize the wavelength of a tunable VCSEL over a range of
temperatures. We demonstrate that it is possible to achieve
wavelength stability concurrently with significant analog
modulation performance improvement. We report a high
spur-free dynamic range (SFDR) of 105.5 dB·Hz2/3 for a 1.55
um VCSEL at 1 GHz, which makes these lasers extremely
attractive for low-cost broadband applications.
II. BACKGROUND
The injection locking technique has been demonstrated to
enhance laser bandwidth, reduce non-linearities [1, 2] and
reduce chirp for direct current digital modulation applications
[1]. These properties can potentially lead to an increase of the
bandwidth-distance product, and for analog applications, lead
to an increase in dynamic range.
The locking phenomenon occurs when a directly-modulated
Manuscript received July 31, 2003.
The authors are with the Department of Electrical Engineering and
Computer Science, University of California, Berkeley, CA, 94720, USA
(phone (510) 642-1023, e-mail: lukasc@eecs.berkeley.edu).
follower laser (also known as slave) is injected by a CW
master laser emitting at a nearly identical wavelength. With a
strong enough injection, the follower laser’s wavelength is
snapped onto the master laser’s wavelength, exhibiting the
most visible sign of locking. The original wavelength
mismatch between the master and follower lasers is called
wavelength detuning. In general, the higher the injection
power, the larger the allowed detuning for stable locking.
With the additional injected photons, the rate equations
governing the follower laser are modified with additional
terms. With an appropriate choice of parameters, the
relaxation oscillation frequency can be increased by a factor
of 3-4 [1-3]. A DFB laser injection locked with another DFB
was demonstrated to have a three times enhancement in the
small signal modulation and reduced dynamic distortions [3].
Recently, we demonstrated the largest locking range (>0.6
nm) for VCSELs with high injection ratio conditions, and
observed an increase of 20 dB·Hz2/3 in the spur-free dynamic
range [1]. SFDR improvement is observed over a very large
detuning range of 0.2 nm to 0.5 nm.
III. EXPERIMENTS
In this experiment, we study the analog performance of
injection-locked VCSELs for ambient temperatures ranging
from 20 to 50°C, demonstrating that the VCSEL wavelength
can be locked onto a WDM channel wavelength through
injection locking.
Furthermore, the performance
improvements remained uniform within this temperature
range. The VCSELs are from Bandwidth 9 Inc., have a tuning
range of ~20 nm and a threshold current of ~2 mA [4].
The experimental setup is shown in Figure 1. The output of
the master DFB laser is injected through a circulator to the
tunable VCSEL. The VCSEL temperature and tuning voltage
are variables in the experiment. The performance of the
tunable VCSEL is measured at the output port of the
circulator, including small signal modulation response (S21)
and spur free dynamic range (SFDR).
Output to detector, analyzer
Injection Master
Laser DFB
Injection-Locked
VCSEL
Fixed bias, temperature
Fixed wavelength
Fixed bias,
Variable temperature
Variable λ tuning voltage
Figure 1 – System Diagram
The DFB is biased at 100 mA and the DFB temperature tuned
at a lasing wavelength of 1545.15 nm. The VCSEL ambient
temperature was controlled by a TEC, in a range of 20°C to
Accepted for publication in IEEE PTL March 2004.
-10
Amplitude (dB)
-20
+0.02nm
+0.08nm
+0.13nm
(-16 GHz)
-0.03nm
-0.06nm
(+7.5 GHz)
-30
-40
-50
-60
Free-running
0
2
4
6
8 10 12 14
Frequency (GHz)
Figure 2 - VCSEL frequency response at 40°C; free-running (3.8 GHz
resonance frequency), and injection-locked (7-14 GHz).
In Figure 2, the calibrated frequency response of the VCSEL
at 40°C for various wavelength detuning values is shown.
The free-running VCSEL shows a resonance frequency (fr) of
nearly 4 GHz, while the injection-locked fr ranges from 7 to
14 GHz. The largest red-side detuning values (ex. -16 GHz or
+0.13 nm detuning) result in the flattest frequency responses,
with as much as 15 dB RF gain at lower frequencies (<4
GHz); this has been observed in many devices. The reason for
the increase (and decrease for small detuning) in modulation
efficiency is still under investigation, and has never before
been predicted or observed.
Two-tone intermodulation distortion (IMD3) as well as singletone third harmonic measurements were performed for 2050°C. The data was collected in a separate experiment than
the S21 data. A typical two-tone measurement, performed at
1.0 GHz (10 MHz tone spacing) is shown in Figure 3 at 50°C.
The noise floor, at –105 dBm with 100 Hz resolution BW,
was limited by the detector noise, rather than laser RIN. The
VCSEL RIN was measured to be -130 dB/Hz, which gives a
RIN-limited noise floor of -145 dBm/Hz; this value is used in
the SFDR calculations. The fundamental and IMD3 powers
were fitted to slopes of 1 and 3 respectively. The spur-free
dynamic range improved from the free-running value of 83.5
dB·Hz2/3 to an injection-locked value 105.5 dB·Hz2/3. A
fundamental tone power increase of 17 dB and a distortion
-40
Injection-Locked SFDR
105.5 dB Hz2/3
-60
-80
83.5 dB Hz2/3
RIN floor
-145 dBm/Hz
-100
-30
-25
-20
Input RF Power (dBm)
-15
-10
Figure 3 – Two-tone SFDR improvement at 50°C, ~0.2 nm detuning and 1.0
GHz modulation. Inner lines (free-running), outer lines (injection-locked).
In Figure 4, the analog modulation performance is shown for
several temperatures as wavelength detuning is varied
(adjusted by the VCSEL tuning voltage). In Figure 4a, the
resonance frequency shows a peak frequency of ~14 GHz for
all temperatures, as well as a broad range of detuning values
(0 to ~0.2 nm) that yield a uniform fr. In Figure 4b, the SFDR
at 30°C is shown to improve for detuning values ranging from
0.1 to 0.2 nm. This range is the most useful for analog
modulation, because it features improved modulation
efficiency, a high and nearly flat frequency response, and an
improved dynamic range. In another laser, this useful locking
range was 0.2-0.3 nm [3]. For detuning values outside the
locking range (approximately -0.1 nm to 0.25 nm), the
VCSEL is unlocked, and the fr and SFDR return to the freerunning values.
170
15 a)
160
20 C
30 C
40 C
50 C
150
10
140
Free-Running
130
5
Injection120
Locked
110
0
SFDR
b)
100
Improvement
90
-5
Locked 30C
80
Free-running
-10
70
-0.1 0.0 0.1 0.2 0.3 0.4 0.5
Detuning (nm)
Two-Tone SFDR
0
-20
Output RF Power (dBm)
The measured frequency responses (S21) were calibrated for
the device and packaging parasitics. The parasitic response
was determined by the method described in [5], using the
theoretical injection-locked frequency response [2] rather than
the typical small-signal modulation response. The difference
of two experimental injection-locked curves is curve-fitted
with the difference of two theoretical injection-locked
responses. The ideal response is then compared to the
measured response, and the difference is the parasitic term.
This procedure was performed several times to find the
average parasitic response.
reduction of 15 dB were observed in this case. The
fundamental tone power increase can be seen in the frequency
response curves in Figure 2, where large detuning values
result in the highest RF gain.
Resonance Frequency (GHz)
50°C, which was limited by the TEC cooler. For each
temperature, the cantilever tuning voltage of the VCSEL was
adjusted yielding a detuning between the master and follower
lasers. Due to packaging thermal expansion limitations,
optical realignment was necessary for each temperature. The
VCSEL bias current was chosen to be fixed at 5 mA, which
gives the peak power for the VCSEL at 50°C.
Figure 4 – Injection-locked VCSEL (a) Resonance Frequency vs. Detuning for
20-50°C. (b) Spur-free dynamic range for 30°C.
For each temperature, an optimized injection-locked two-tone
SFDR is compared to the free running references, shown in
the top two curves in Figure 5. Although in this experiment,
the wavelength was not optimized, and thus the SFDR or fr
values are not the highest. The free-running SFDR degrades
with higher temperature, while the injection-locked SFDR
remains reasonably uniform between 98-103 dB·Hz2/3, with an
improvement ranging from 8-20dB·Hz2/3. The bottom curves
are the resonance frequencies for free-running and injectionlocked cases (at 0.1 nm detuning). The free-running fr
Accepted for publication in IEEE PTL March 2004.
Free-running
12
90
80
Injection-locked
70
Free-running
20
25
30 35 40 45
Temperature (C)
8
4
50
0
Figure 5 – Two-tone (1 GHz) SFDR vs. temperature for both free running and
injection locked cases (top curves). Resonance Frequency: free-running and
injection locked (bottom curves)
Figure 6 shows the necessary tuning voltage to be applied to
the VCSEL, for ambient temperatures 15-50°C. In the figure,
the fr and SFDR improvement ranges are shown. A
bandwidth improvement is typically seen for a ±0.3V voltage
range, while the SFDR improvement occurs over a ±0.1V
range. Importantly, the region where the SFDR improves
overlaps the fr improvement region, and is for high red-side
detuning values, where the S21 shows a modulation efficiency
improvement and flat frequency response.
IV. PROPOSED INJECTION-LOCKED TRANSMITTER
We propose a novel architecture that exploits the injection
locking technique to lock an array of tunable lasers to the ITU
grid, while simultaneously improving the modulation
performance. One temperature controlled reference laser
would lock an array of uncooled VCSELs. Each VCSEL
tuning voltage would be adjusted using a look-up table, such
that both SFDR and S21 improve, and the wavelength is
locked to the ITU grid. Because the improvement range is
relatively large, some error in the detuning control is tolerable
(±0.1V) and the temperature reading does not have to be
accurate. For applications requiring a constant optical output,
both the bias and tuning voltage would be adjusted using a
look-up table for varying temperatures. Note that in this
study, the constant biasing scheme resulted in the output
power varying with temperature.
A master laser with many optical modes could be used to
injection-lock several VCSELs simultaneously. One could
employ a stable mode-locked laser (MLL) [6] or an optical
frequency comb generator [7]. The mode spacing for the
mode locked laser can be designed to match the ITU WDM
grid. The output of the mode locked laser is sent into a
demultiplexer following a circulator, and each optical finger
feeds and locks one laser, similar to our experimental setup.
Because of the stable output from the mode locked laser, we
expect that this 1-N injection locking architecture will have
similar performance as to the experiment where each follower
laser is injection-locked by its own master laser. In this
application of the injection locking technique, only one
temperature and wavelength controller will be required for the
master laser, rather than for each transmitter.
23
Tuning Voltage
16
100
60
Resonance Frequency (GHz)
Spur-Free Dynamic Range
decreases with increasing temperature, while the locked fr
remains nearly constant at ~8 GHz. This demonstrates that
the injection-locked analog performance is highly improved
and nearly uniform for the temperature range studied.
20
110
Injection-locked
22
Resonance Frequency
Enhancement
21
20
SFDR Enhancement
19
10
20
30
Temperature
40
50
Figure 6 – VCSEL Tuning voltage range for S21 and SFDR improvement.
V.
CONCLUSION
In this work, we proposed an uncooled VCSEL transmitter for
WDM applications. We report the highest SFDR of 105.5
dB-Hz2/3 achieved for a VCSEL at 1 GHz. We demonstrate
that with a continuously tunable VCSEL, it is possible to use
the tuning voltage to ensure wavelength locking and
performance enhancements through temperature variation.
This demonstrates the feasibility of uncooled VCSEL
transmitters injection-locked to a WDM reference grid.
ACKNOWLEDGMENTS
The authors thank Peter Schultz and Rajesh Patel at MIT
Lincoln Labs for their assistance with the RIN measurements.
This work was performed with funding from DARPA CSOM
Award F30602-02-2-0096.
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