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High-Speed Photodetector Characterization Using Tunable
Laser by Optical Heterodyne Technique
Haisheng Sana,b, Jimin Wena,
Liang Xiea, Ninghua Zhu*a, Boxue Fengb
aState Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors,
Chinese Academy of Sciences, P.O.Box 912, Beijing, China 100083;
bKey Laboratory for Magnetism and Magnetic Material of Ministry of Educationˈ
Lanzhou University, Tianshuinan Road 222, Lanzhou, Gansu, China 730000
ABSTRACT
An ultra-wide-band frequency response measurement system for optoelectronic devices has been established using the
optical heterodyne method utilizing a tunable laser and a wavelength-fixed distributed feedback laser. By controlling the
laser diode cavity length, the beat frequency is swept from DC to hundreds GHz. An outstanding advantage is that this
measurement system does not need any high-speed light modulation source and additional calibration. In this
measurement, two types of different O/E receivers have been tested, and 3 dB bandwidths measured by this system were
14.4GHz and 40GHz, respectively. The comparisons between experimental data and that from manufacturer show that
this method is accurate and easy to carry out.
Keywords: Optical Heterodyne Technique, Ultra-Wideband Frequency Response, Different Frequency, Photodetector
1. INTRODUCTION
With the development of digital optical communication system, the measurement of fast optical waveform is one of the
important techniques used, not only for basic research, but also for practical application. Recently, more and more
researches have been focused on ultra-high speed photo-electronic devices, such as high-speed and high sensitivity
optical receiver and high-speed modulator and so on. Up to now, the p-i-n detector (PD) reported could obtain sensitivity
of 0.85 A/W and bandwidth of 50 GHz1 and sensitivity of 1.02 A/W and bandwidth of 48 GHz 2. As the bandwidth of
these detectors reaches 50 GHz, it become increasingly difficult to measure their frequency response. The traditional
measurement methods, such as electro-optic modulation method 3,4, short pulses method 5, FM sidebands method 6 and
white noise method 7, however become increasingly difficult to accurately measure the bandwidth more than 20GHz 8,9,
primarily because it is difficult to separate the response of the photodetector from the response of a modulated source
used to test the detector.
Presently, the optical heterodyne technique utilizing coherence of light has been proved to be accurate and reliable in
measuring ultra-wideband frequency response 8,10,11. Especially in recent years, the fast development of wavelength
*nhzhu@red.semi.ac.cn; phone +86-10-82304385; fax +86-10-82304385
Invited Paper
Optoelectronic Materials and Devices for Optical Communications,
edited by Shinji Tsuji, Jens Buus, Yi Luo, Proc. of SPIE Vol. 6020,
60200A, (2005) · 0277-786X/05/$15 · doi: 10.1117/12.636311
Proc. of SPIE Vol. 6020 60200A-1
tunable lasers decrease the complexity of heterodyne measurement system and enable the method easily implemented.
An advantage of optical heterodyne technique is that measurement system could use a low-speed optical source,
allowing for a straightforward measurement of PD frequency response with only an instrumental limit to the highest
frequencies that can be measured.
2. THEORY
In optical heterodyne detection, the mixed optical intensity of two single frequency laser beams with a frequency
difference
Q
b is given by
12 12
2b
I
tII cos IIcost
M
Q
˄1˅
The photocurrent ic(t), therefore, is written as
>@
12 12
2
cb
e
it I I F cos II cos t
hv b
K
ZM Q
˄2˅
Here I1and I2 are the received optical intensities and
M
is the angle between polarized directions of the two beams, e is
the electron charge,
K
is quantum efficiency, hv is photon energy, and F(
Q
b) is frequency response of optical receiver.
The last item of equation corresponds to the different frequency signal with a frequency
Q
b.
Therefore, the signal power measure in a spectrum analyzer can be expressed as follows 8
22
22 2 2
12 2
22
2
22 2 2
12
2
22
2
b
b
B/
bSb
B/
b
Sb
e
PEERcosF
hv
eB
E E R cos F arctan
hv
Q
Q
KQ
d
Q
M
QQ
Q
SQQ
KMQ Q
'
'
ª
«
¬¼
º
»
'
³˄3˅
Here RS=50 :is the input impedance of the spectrum analyzer, B is the resolution bandwidth of spectrum analyzer, and
and '
Q
is the linewidth ( FWHM ) of the different frequency signal and equals to the sum of the linewidths of two light
beams.
It can been seen from Equation (3) that, If laser output I1and I2, polarization difference
M
and linewidth '
Q
can be kept
constant, the frequency response of the PD F(
Q
b) can be directly measured by the different frequency signal level in a
spectrum analyer.
Proc. of SPIE Vol. 6020 60200A-2
Tunable external cavity laser
Peratcomputer
Frequency
Beam separater SMF
)lerE7f
I
Isolator' Optical
Polarization SMF iSpectrometer
Peltier unit Controller
II
1.Sum DFB batterily-packaging laser
Dihedral reflector
Collimating
Lens
3. EXPERIMENTAL SETUP
Fig.1 is the Schematic diagram of optical heterodyne measurement system used in this experiment.
Fig. 3 Schematic diagram of frequency response measurement system
based on optical heterodyne technique
The reference optical source used in this system is 1550 nm distributed feedback (DFB) butterfly packaging laser, Since
the temperature dependence of frequency for DFB-LD is about 11 GHz/deg, a Peltier cooling element was adopted for
avoiding the wavelength-shift introduced by temperature. To prevent any optical feedback toward the cavity, a beyond
42 dB optical isolator is inserted in the laser output beam. The sweep optical source used is TUNICS-purity SC tunable
external cavity laser manufactured by NetTest company, the laser tuning characteristics are shown in Table 1.
Table 1 TUNICS-purity tunable laser characteristics
Name Value Unit
Wavelength Range 1465-1570 nm
Power Stability f0.01 dB
Absolute Wavelength Accuracy f0.2 nm
Linewidth (coherent off) 150 kHz
Wavelength Seting Resolution 0.001 nm
Optical Frequency Fine Tuning f2.0 GHz
Signal to Source Spontaneous Emission Ratio !90 dB
Relative Intensity Noise (RIN) !145 dB/Hz
Proc. of SPIE Vol. 6020 60200A-3
IO.Gi -
-90.01549.30 1549.795 1550.29
WaeIcngth (nm)
(b)
o2O25Frequency (GHz)
The optical layout of the cavity conforms to a modified Sagnac interferometer configuration. Within the cavity, the
double-pass reflection on the grating provides maximum dispersion, which maximizes the mode spacing, yields a
spectrally pure and truly single-mode operation, the collimating lens, optical isolator and dihedral rear reflector override
any small misalignments and guarantee the output light stability.
In this experiment, the optical spectrums and different frequency spectrums are detected by Advantest company’s Q8384
optical spectrum analyzer and R3182˄9KHz a 40GHz˅frequency spectrum analyzer, respectively. A measurement of
frequency response for a HP11982A O/E converter with 3dB bandwidth of about 15GHz and a New Focus Model 1014
optical detector of 45GHz bandwidth is respectively carried out by using this system and give the measurement results.
4. EXPERIMENT RESULTS AND DISCUSSION
When the output optical wavelength of reference optical source is fixed at 1549.793 nm, and that of tunable source is
tuned at 1549.886 nm, the coupling-out light spectrum and different frequency spectrum detected by using the
measurement system are shown in Fig.2, respectively.
Fig. 2 The coupling-out optical spectrum (a) and the different frequency spectrum (b).
Since the different frequency can be evaluated by
12
2
1212
11cc
fc
OO
O
OO OO O
' | '
(4)
The different frequency calculated by equation (4) is 11.7 GHz that agree well with the center frequency of signal shown
in Fig.2(b).
Proc. of SPIE Vol. 6020 60200A-4
(a).
•—s—Peak trace? (Va:
—0— Relative response F
0 3 6 9 12 15 18 21
Frequency (6Hz)
(b)
OoO'OOb%doo/o °c0
\.'.:zv •'i•
—•—PeaktraceP(;)
—0— Relative response F (vb)
0 5 10 15 20 25 30 35 40
Frequency (GHz)
Fig. 3 Frequency response of photodetector tested
(a) HP11982A and (b) New Focus Model 1014.
Before measuring PD frequency response, the frequency spectrum analyzer first is calibrated to the input port of coaxial
cable (bandwidth of 40 GHz). The resolution bandwidth (RBW) and video bandwidth (VBW) of frequency spectrum
analyzer are set as 3MHz and 3 KHz, respectively. Additionally, a polarization controller is used in one of two input
arms to increase the system sensitivity.
The frequency response of the HP11982A O/E converter is show in Fig.3 (a), the trace recorded with the solid circles
denote actual measurement peak values P(vb). According to equation (3), the converter response F(vb) represented with
hole circle can be obtained by extracting root (half of dB value) for peak values P(vb). It can be seen from Fig.3 (a) that
the 3dB bandwidth of converter is 14.4 GHz that is nearly coincident with known bandwidth. Likewise, the
measurement results for New Focus Model 1014 photodetector are shown in Fig.3(b), the step is set as 0.02 nm, and
sweep range cover through 40 GHz. It can be seen from Fig.3 (b) that the fluctuation range of response curve is
controlled within 3 dB, thus we can decide that 3dB bandwidth of photodetector is greater than 40 GHz.
For decreasing the random error and operation load, the measurement can be automatically implemented by connecting
computer to GPIB interface of laser and using the peak-hold function of frequency spectrum analyzer.
For the ideal measurement of a photodetector response, this peak search algorithm requires a spectrum analyzer with a
RBW larger than the spectral width of different frequency signal. Thus, for increasing the measurement accuracy, the
RBW should be set at maximum value, and a more narrow linewidth laser should be used.
5. CONCLUTION
An ultra-wide-band frequency response measurement system for photodetector has been established using the optical
heterodyne detection method. By tuning the laser wavelength length, the different frequency is swept from DC to
hundreds GHz. An outstanding advantage is that this system does not need any high-speed light modulation source and
additional calibration. Experiments results show that this method is accurate and easy to carry out, especially adapting to
the frequency response measurement of high-speed photoelectronic devices.
Proc. of SPIE Vol. 6020 60200A-5
ACKNOWLEDGEMENTS
The authors would like to thank L. J. Zhao and J. L. Gao of Photoelectronic Process Center for supplying tunable
laser and some measurement instruments and H. Q. Yuan and Y. Liu, also of Photoelectronic Process Center, for DFB
laser diode packaging. Additionally, this work was supported by the Natural Sciences Foundation of China. (No.
60510173).
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Proc. of SPIE Vol. 6020 60200A-6
