IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005 2415
Optically Injection-Locked Self-Oscillating
Optoelectronic Mixers Based on InP–InGaAs HPTs
for Radio-on-Fiber Applications
Chang-Soon Choi, Jae-Young Kim, Woo-Young Choi, Member, IEEE, Hideki Kamitsuna, Member, IEEE,
Minoru Ida, and Kenji Kurishima
Abstract—We demonstrate an optically injection-locked self-os-
cillating optoelectronic mixer (OIL-SOM) for radio-on-fiber
downlink applications. OIL-SOM is based on a monolithic oscil-
lator containing an InP–InGaAs heterojunction phototransistor
(HPT). The oscillator is OIL by remotely delivered optical local os-
electrical LO signals. In addition, the HPT in this oscillator can
simultaneously perform optoelectronic mixing, in which optically
transmitted intermediate frequency signals are up-converted to
the desired radio-frequency band with high conversion efficiency.
With these, OIL-SOM can make base station architecture simple
in radio-on-fiber downlink systems.
(OIL) oscillator, optoelectronic mixer, phase-locked oscillator,
networks because they offer an efficient way to merge fiber-
optic networks with broad-band wireless communication sys-
tems , . With optical fiber having low loss and wide band-
width transmission characteristics, it is possible to distribute
broad-band data and/or high frequency signals to many wire-
line or wireless subscribers.
Among several approaches for realizing radio-on-fiber sys-
tems, a remote up-conversion scheme has received much atten-
tion because it provides immunity to dispersion-induced carrier
suppression problems and compatibility with wavelength-divi-
sion-multiplexing fiber-optic networks , . However, com-
plex base station architecture is inevitable because many com-
ponents are required in the base station such as photodetector,
sistors based on indium phosphide (InP) materials are useful for
simplifying base station architecture because they can simulta-
neously perform several functions such as photodetection and
ADIO-ON-FIBER systems are expected to play an im-
portant role in broad-band wireline/wireless convergence
Manuscript received July 5, 2005; revised July 29, 2005. This work was sup-
ported by the Ministry of Science of Technology of Korea through the National
Research Laboratory Program.
C.-S. Choi, J.-Y. Kim, and W.-Y. Choi are with the Department of Electrical
and Electronic Engineering, Yonsei University, Seoul 120-749, Korea (e-mail:
email@example.com; firstname.lastname@example.org; email@example.com).
H. Kamitsuna, M. Ida, and K. Kurishima are with the NTT Photonics
Laboratories, NTT Corporation, Kanagawa 243-0198, Japan (e-mail: kami-
firstname.lastname@example.org; email@example.com; firstname.lastname@example.org).
Digital Object Identifier 10.1109/LPT.2005.857624
optoelectronic mixing. Furthermore, they are fully compatible
making one-chip solution for the entire base station excluding
antenna a possibility –.
However, the realization of low-cost and miniaturized phase-
locked oscillator is a challenging task especially for millimeter-
wave applications. One attractive solution for this problem is to
optically distribute LO signals from the central office to many
base stations , . In such a scheme, data can be delivered
from central office to base station in optical intermediate fre-
quency (IF) signals and frequency up-converted to the desired
radio-frequency (RF) band at the base station through mixing
with optical LO signals. Although this scheme has the potential
to eliminate phase-locked oscillators in many base stations, the
total amount of required optical LO power can be very high in
order to support efficient frequency up-conversion process and
the optical LO power deliveredto each base station can vary de-
pending on the fiber transmission distance, causing difficulties
in system design.
In this letter, we propose new base station architecture based
on optically injection-locked self-oscillating optoelectronic
mixer (OIL-SOM) which can solve the above-mentioned prob-
lems. It simultaneously performs photodetection to 1.55- m
lightwave, frequency mixing, and phase-locked oscillation. Be-
cause it can be realized with a monolithic oscillator integrated
circuit (IC), a very simplified base station is possible for remote
up-conversion radio-on-fiber systems.
II. PROPOSED SCHEME
Fig. 1 schematically shows the proposed base station archi-
tecture based on OIL-SOM. A simple free-running oscillator
based on three-terminal phototransistor can be used for real-
izing OIL-SOM. The free-running oscillator is injection-locked
by optically delivered LO signals from the central office. At the
same time, optical IF signals at different wavelengths from op-
tical LO are illuminated to the phototransistor inside the oscil-
lator and frequency up-conversion can be achieved by optoelec-
tronic mixing. For example, RF spectrum for OIL-SOM based
on InP–InGaAs heterojunction phototransistor (HPT) oscillator
IC is shown in the inset of Fig. 1 when optical LO frequency
and IF are 10.94 GHz and 200 MHz, respectively.
Using this OIL-SOM, base station architecture for remote
up-conversion scheme can be significantly simplified because
a single oscillator plays the roles of photodetector, frequency
1041-1135/$20.00 © 2005 IEEE
2416IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005
example of its output spectrum.
Proposed antenna base station architecture based on OIL-SOM and
InP–InGaAs HPT oscillator IC. MZM: Mach–Zehnder modulator.
Experimental setup for OIL-SOM and the schematic diagram for
mixer, and high-power phase-locked LO. Because the output
power of OIL oscillator depends not on incident optical LO
power but on the free-running oscillator itself , OIL-SOM
allows both high LO power and efficient frequency up-conver-
sion independent of optical transmission distance.
III. EXPERIMENTAL RESULTS
The detailed description of InP–InGaAs HPT oscillator used
in our investigation can be found in . The device structure of
HPT is identical to that of InP–InGaAs heterojunction bipolar
5- m diameter is located on top of emitter. The HPT is fully
compatible to InP HBT-based MMIC process, and a monolithic
OIL-SOM can be easily realized. The electrical current gain
and maximum frequency of oscillation
for the HPT are 153 and 94 GHz, respectively. The fab-
at 10-GHz optical modulation frequency, which is expected to
provide a wide locking range of optical injection-locked oscil-
Fig. 2 shows the circuit diagram for 10-GHz band os-
cillator IC used in our investigation. The chip size is only
0.54 mm. The common emitter feedback was realized
by using spiral inductors, MIM capacitors, and an HBT acting
as a variable resistor. Free-running oscillation frequency and
quality factor of the oscillator can be controlled by adjusting
bias voltage for the HBT
. An external bias-tee was used
InP–InGaAs oscillator is under (A) free-running (B) OIL (?
and (C) unlocked (?
characteristics for ?
and injection-locked conditions are compared in (D).
? ?? dBm),
??? dBm) conditions. The phase-noise
when the oscillator is under free-running
for collector biasing of the HPT
for characterizing OIL-SOM is also shown in Fig. 2. Optical
LO signals at 1550 nm were generated by a Mach–Zehnder
modulator and amplified by an erbium-doped fiber amplifier.
For optical IF signals, a distributed feedback laser diode having
lasing wavelength of 1552 nm was directly modulated with
200-MHz IF signals. For simplicity, no data were applied to IF
signal. Optical LO and optical IF signals were combined and
illuminated onto the HPT in the oscillator through single-mode
lensed fiber. During these measurements, optical IF power
injected to the HPT was set to be
To investigate the influence of optical injection-locking
on the quality of frequency up-converted signals,
signals were observed at the output port of InP–InGaAs HPT
oscillator. Fig. 3 shows the output RF spectra at
when the oscillator is under (A) free-running, (B) OIL, and
(C) unlocked conditions when
was judged injection-locked when its output spectrum has
the same optical modulation frequency with low phase-noise.
When OIL, the frequency up-converted signals exhibit very
stable spectrum and suppressed phase-noise characteristics. We
obtained a very wide locking range of about 1.4 GHz under
the injection of 2-dBm optical LO signals. Fig. 3(D) shows the
phase-noise characteristics of frequency up-converted signals
when the oscillator is free-running and OIL. Phase noises of
frequency up-converted signals are significantly reduced by
optical injection-locking, resulting in phase-noise values of
88.5 and104 dBc/Hz at 10- and 100-KHz frequency offsets
, respectively. Such phase-noise characteristics
should be sufficient for many wireless applications employing
phase-modulated data schemes. When incident optical LO
power is lower than
10 dBm, the oscillator goes into unlocked
. The experimental setup
V. The oscillator
CHOI et al.: OIL-SOM BASED ON InP–InGaAs HPTs FOR RADIO-ON-FIBER APPLICATIONS2417
input optical LO powers.
Measured phase-noises and internal conversion gains as a function of
condition and frequency up-converted signals have many spu-
rious sidebands originating from frequency mixing of optically
injected signals and free-running oscillation signals.
The phase-noise characteristics of frequency up-converted
signals were measured as a function of input optical LO power,
and the results are shown in Fig. 4. On the condition that optical
LO power is higher than
8 dBm, phase-noise characteristics
are independent of optical LO power. Reducing optical LO
8 dBm makes the phase-noise characteristics
deteriorated even under OIL condition. The internal conversion
gain of OIL-SOM is also shown in Fig. 4 as a function of input
optical LO power. The internal conversion gain is defined as
the power ratio of the optoelectronic mixing signal at
to the primary photodetected signal at
when HPT is at cutoff. As can be seen in Fig. 4, high internal
conversion gain of about 9 dB can be achieved regardless of
input optical LO power. These features are very attractive for
radio-on-fiber systems that employ optical LO distribution
scheme because frequency up-conversion performance is not
significantly influenced by the amount of optical LO power,
which may vary among base stations, as long as the locking
condition is achieved with relatively small optical LO power of
that can be measured
We investigated OIL-SOM that can be used for simplifying
base station architecture in remote up-conversion radio-on-fiber
downlink systems. It provides low phase noise and high output
power LO signals and simultaneously perform optoelectronic
over the wide power range of optical LO, which promises high
performance independent of optical transmission distance. Be-
cause this approach is based on a single InP–InGaAs HPT os-
cillator, it can greatly reduce the complexity of base station ar-
chitecture in radio-on-fiber systems.
H. Kamitsuna, M. Ida, and K. Kurishima would like to thank
Dr. Y. Itaya, Dr. M. Muraguchi, Dr. H. Sugahara, Dr. T. Enoki,
and Dr. K. Murata at NTT Photonics Laboratories for their sup-
port and encouragement.
 A. J.Seeds,“Microwavephotonics,”IEEETrans.Microw.Theory Tech.,
vol. 50, no. 3, pp. 877–887, Mar. 2002.
 L. Noel, D. Wake, D. G. Moodie, D. D. Marcenac, L. D. Westbrook,
and D. Nesset, “Novel techniques for high-capacity 60 GHz fiber-radio
transmission systems,” IEEE Trans. Microw. Theory Tech., vol. 45, no.
8, pp. 1416–1423, Aug. 1997.
 E. Suematsu and N. Imai, “A fiber-optic/millimeter-wave radio trans-
mission link using HBT as direct photodetector and an optoelectronic
up-converter,” IEEE Trans. Microw. Theory Tech, vol. 44, no. 1, pp.
133–143, Jan. 1996.
 U. Gliese, S. Norskov, and T. N. Nielsen, “Chromatic dispersion in
fiber-optic microwave and millimeter-wave links,” IEEE Trans. Microw.
Theory Tech., vol. 41, no. 10, pp. 1716–1724, Oct. 1996.
 H. Kamitsuna, T. Shibata, K. Kurishima, and M. Ida, “Direct optical
injection-locking of InP/InGaAs HPT oscillator ICs for microwave pho-
tonics and 40 Gbps-class optoelectronic clock recovery,” IEEE Trans.
Microw. Theory Tech., vol. 50, no. 12, pp. 3002–3008, Dec. 2002.
 C.-S. Choi, H.-S. Kang, W.-Y. Choi, D.-H. Kim, and K.-S. Seo, “Pho-
totransistors based on InP HEMTs and their applications to millimeter-
wave radio-on-fiber systems,” IEEE Trans. Microw. Theory Tech., vol.
53, no. 1, pp. 256–263, Jan. 2005.
 H. Kamitsuna, Y. Matsuoka, S. Yamahata, and N. Shigekawa, “Ultra-
high-speed InP/InGaAs DHPT for OEMMIC,” IEEE. Trans. Microw.
Theory Tech., vol. 49, no. 10, pp. 1921–1925, Oct. 2001.
 M. Tsuchiya and T. Hoshida, “Nonlinear photodetection scheme and
its system applications to fiber-optic millimeter-wave downlinks,” IEEE
Trans. Microw. Theory Tech., vol. 47, no. 7, pp. 1342–1350, Jul. 1999.
 Y.-K.Seo,C.-S.Choi,andW.-Y.Choi,“Allopticalsignal up-conversion
for radio-on-fiber applications using cross gain modulation in semicon-
ductor optical amplifier,” IEEE Photon. Techol. Lett., vol. 14, no. 10, pp.
1448–1450, Oct. 2002.