IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000
An Integrated Hybrid Device for Binary-Phase-Shift-Keying Subcarrier Modulation M. Shin, J. Lim, C. Y. Park, J. Kim, J. S. Kim, K. E. Pyun, and S. Hong
Abstract—We propose an integrated optical/radio frequency (RF) hybrid device for binary-phase-shift-keying subcarrier modulation that is based on optical amplitude modulation and interference with phase delay. The device consists of two multiple-quantum-well (MQW) electroabsorption (EA) modulators branched with two multimode interference (MMI) couplers. When an RF carrier was applied to one modulator and a digital signal to the other one, the phase modulation of the RF subcarrier was realized. Index Terms—BPSK, MMI coupler, MQW electroabsortion modulator, subcarrier.
ICROWAVE and millimeter (mm) waves are receiving increasing attention since these high frequencies are expected to provide wide-band channels required for both mobile communication system and wireless local area networks . The digital transmission systems with microwave and mm wave using QPSK or BPSK modulation have been reported , . To realize such systems, an approach of optical and radio frequency (RF) hybridization is proposed as important solution. A hybrid fiber/coax network allows robust transmission of RF subcarrier signal via a single mode fiber to the fiber node, and thus the significant reduction in the number of cascade RF amplifiers. Although mm-wave subcarrier bands have the capacity of higher than 1 Gb/s, most of the previous works reported at most a few hundred megabits per second , . This is due to the lack of proper modulation scheme for the hybrid approach. In general, upconversion schemes in the transmitter use an intermediate frequency to convert base-band signal to mm-wave band, and so the data rate cannot exceed the intermediate frequency. Also needed are high-frequency amplifiers and mixers that are very difficult to implement if one wants to utilize the full capacity of mm-wave carriers. Recently,
Fig. 1. The proposed device configuration. The optical phase is different by at the out-of-phase output port.
a hybrid modulation scheme for phase-shift keying (PSK) based on optical delay switching is proposed and demonstrated . In this letter, an integrated hybrid device that can directly generate a BPSK-modulated subcarrier is proposed. The device consisted of two high-speed MQW EA modulators branched with two MMI couplers. Since the subcarrier and the digital data are introduced independently to the modulators, the subcarrier frequency and the data rate are limited only by the bandwidth of high-speed EA modulators , . Therefore, the direct upconversion from baseband to mm-wave band is expected to be possible. The BPSK-modulated subcarrier is observed at the output using both the optical amplitude modulation of modulators and the interference due to phase delay of MMI couplers. II. THE DEVICE AND FABRICATION
Manuscript received November 15, 1999. This work was supported in part by the Korea Science and Engineering Foundation (KOSEF) through the OptoElectronics Research Center (OERC-99-0809-02-02) at the Korea Advanced Institute of Science and Technology (KAIST) and the Ministry of Information and Communications at ETRI. M. Shin and J. Lim are with the Telecommunication Basic Research Laboratory, ETRI, Taejon 305-600, South Korea, and with the Department of Electrical Engineering, KAIST, Taejon 305-701, South Korea (e-mail: [email protected]
). C. Y. Park was with the Telecommunication Basic Research Laboratory, ETRI, Taejon 305-600, South Korea. He is now with the ETRI TBI Center, XL Photonics, Inc., Taejon 305-333, South Korea. J. Kim, J. S. Kim, and K. E. Pyun are with the Telecommunication Basic Research Laboratory, ETRI, Taejon 305-600, South Korea. S. Hong is with the Department of Electrical Engineering, KAIST, Taejon 305-701, South Korea. Publisher Item Identifier S 1041-1135(00)03620-X.
The device structure is illustrated in Fig. 1. The input 3-dB MMI coupler divides and branches an incident light to two equal MQW EA modulators. The MMI coupler is designed to have a mirror image at an output using paired interference . Thus, phase diftwo optical outputs have the same amplitude but ference. Since these input/output MMI couplers, which are designed to be the same, are connected to the optical modulators, one output of the output MMI coupler becomes in-phase port, where the optical fields interfere constructively and the other does out-of-phase port, where the optical fields interfere destructively. On that account, the in-phase output is normally on and out-of-phase output is normally off when the both modulators are on. The EA modulator alters the optical amplitude in
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SHIN et al.: INTEGRATED HYBRID DEVICE FOR BPSK SUBCARRIER MODULATION
each branch. If the modulator is turned off, the optical amplitude at the out-of-phase port is directly proportional to that . If the modulator is turned on, through the modulator the optical amplitude at the out-of-phase port is related to the and difference of the optical amplitudes through modulator . and are identical If it is assumed that the modulator and the optical field intensity of each modulator is , the optical field at the out-of-phase port is represented as
is turned ON is turned OFF
Fig. 2. The output images at the MMI outport: (a) the constructive interference at the in-phase output when the two modulators were on and (b) the divided powers at the two output ports when one of the modulators was off.
and are modulated optical amplitudes at modwhere and , respectively. If a digital on-and-off signal ulators and a sinusoidal signal of (0 or 1) is applied to the modulator and frequency is applied to the modulator , amplitude then the optical amplitude at the output in (1) can be written as follows:
is turned ON
is turned OFF (2)
is changed so that the Note that the sign of the amplitude is shifted phase of sinusoidal signal generated at modulator . by in accordance with the state of modulator The proposed device was fabricated in the following process. The epitaxial layers of the modulator region were prepared on n -InP substrate by using a metal–organic chemical vapor deposition (MOCVD). The multiple quantum well (MQW) absorption layers of 250-nm thickness were grown between graded index clads of 70-nm thickness. The MQW layers consisted of m and 0.6% 14 pairs of 1.52 -InGaAsP wells ( tensile strain) with 10-nm thickness and 1.2 -InGaAsP barriers m and 0.8% compressive strain) with 6.5-nm thick( ness. The low-loss passive region was grown by butt-coupled regrowth. Deep dry etching was applied to form the waveguide structures including MMI couplers and MQW EA modulators. A 3- m-thick polyimide was employed to block the leakage current and to reduce the pad capacitance. III. MEASUREMENT RESULTS The characteristics of MMI couplers and EA modulators strongly depend on the wavelength and polarization of light. We used an HP8168F tunable laser and controlled the polarization with a fiber polarizer. These adjustments were monitored by a charge-coupled device (CCD) camera. A 1.53- m light of TE mode was selected to operate the MMI couplers in the optimum condition and to have a large on–off ratio of the modulators. The incident power of light was 5 mW. Each modulator was turned off when a reverse voltage of 4 V is applied. Fig. 2(a) shows that the out-of-phase output is normally off while the in-phase output is normally on. When the voltage of 4 V is
Fig. 3. Normalized output power at the out-of-phase port as a function of the applied carrier signal voltage.
applied to the modulator (OFF state), the light from the splits into two at the output MMI couplers. other modulator Thus, two output ports have the same optical power as can be seen in Fig. 2(b). The out-of-phase output decreases as . On the contrary, when the reverse voltage is increased to is biased with 0 V (ON state), the out-of-phase output . This is because when increases with the reverse voltage at is on, the difference between the optical outputs from two modulators, which appears at the out-of-phase port, increases becomes opaque. We measured the over all modulation as characteristics by using an optical power detector. Fig. 3 shows that the sign of the optical signal modulation with respect to the is changed by the bias voltage of . reverse voltage of The phase of output signal is then changed by in comparison with that of input signal. A 4-V (peak-to-peak) sinusoidal signal of 5 MHz to the modulator was driven as a carrier and a 4-V 500-kHz rectangular was driven directly as a base-band digital signal. signal to In Fig. 4, the output signal detected by a photodiode was scaled to compare the RF source signal. The phase change of the RF subcarrier was observed. As for the EA modulator, we measured the optical and electrical properties of the EA modulators in this device. The on–off ratio at 3 V was 16 dB for TE mode and 13 dB for TM mode at 1.53- m wavelength. The 3-dB bandwidth of the modulators was more than 20 GHz .
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 5, MAY 2000
as an integrated device that has two MQW EA modulators with two MMI couplers. The device can be directly used to upconvert a baseband to an RF subcarrier by optical modulation and interference by phase delay.
Fig. 4. BPSK modulation of 5-MHz carrier by 500-Kbps base-band signal: (a) the RF subcarrier, (b) the modulated optical output, and (c) the base-band signal.
If an RF signal and a digital signal are applied to and , respectively, the output signal at the out-of-phase port becomes a BPSK-modulated subcarrier. This implies a direct upconversion from the baseband to the RF subcarrier. Instead of using the gigahertz range subcarrier and the higher data rate, only the working principle is demonstrated here with the carrier frequency of 5 MHz modulated by a 500-kHz digital signal. Since the working frequency and the data rate are expected to be limited only by the bandwidth of modulator, one can readily extend this operation principle to the mm-wave subcarrier and the data rate of its full bandwidth capacity. In addition to these high-speed advantages, the synchronization of local oscillator for RF subcarrier generation and digital signal is not necessary in this scheme. This is because this modulation mechanism makes the signal phase change at arbitrary time by optical interference. IV. CONCLUSION An optical/RF hybrid device that can efficiently generate a BPSK-modulated subcarrier is proposed. This is implemented
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