Hideo Kuwahara

Efficient coupling from semiconductor lasers into single-mode fibers with tapered hemispherical ends

An efficient coupling from semiconductor lasers in the 1.3-microm range into single-mode fibers with tapered hemispherical ends (TH fiber) is described. Coupling efficiency of more than 35% is attainable with TH fiber coupling. Optimum radius of the end hemisphere is ~20 microm and is obtained with good reproducibility by drawing the fiber in an arc discharge. It was found that changes in laser characteristics due to reflected light are far less for TH fiber coupling than for flat-end coupling. Stable coupling modules using TH fiber coupling were fabricated for system use.

Modulation and demodulation techniques in optical heterodyne PSK transmission systems

Modulation and demodulation techniques are described for an optical PSK heterodyne transmission system operating at 560 Mb/s and 1.2 Gb/s. Performance limitations affecting the receiver sensitivity in a 1.2-Gb/s DPSK system, such as laser phase noise, phase modulation depth, IF center frequency deviation, and local laser power, are studied. High receiver sensitivities for PSK systems were achieved. The applicability of the Mach-Zehnder modulator as a phase modulator for 1.2-Gb/s DPSK is also demonstrated. A 1.2-Gb/s DPSK transmission of over 100 km, using polarization diversity with novel polarization-insensitive automatic frequency control in an attempt to overcome signal fading caused by polarization fluctuation in the transmitting fiber, is also described. A receiver sensitivity of less than -42.8 dBm and varying within 1.4 dB for all states of polarization was achieved. A multichannel high-definition TV (HDTV) transmission experiment using a DPSK polarization-diversity tunable receiver is described. >

Compensation of pulse shape distortion due to chromatic dispersion and Kerr effect by optical phase conjugation

Pulse shape distortion due to chromatic dispersion and self-phase modulation in a single-mode fiber was effectively compensated for by using an optical phase-conjugate wave generated by nondegenerate forward four-wave mixing in a zero-dispersion single-mode fiber. Using optical phase conjugation at the midpoint of a 100-km standard single-mode fiber compensates for the distortion of 10-Gb/s intensity-modulated NRZ pulse at an input power level exceeding +10 dBm with a resultant power penalty of less than 1.2 dB.<<ETX>>

Optimal 40 Gb/s modulation formats for spectrally efficient long-haul DWDM systems

Three modulation formats are compared by numerical simulation of highly dense (75-GHz-spaced for 40 Gb/s channel), long-haul (600-1800 km) wavelength division multiplexed systems with three fiber types. Nonreturn-to-zero (NRZ) format, being the most spectrally compact and the simplest in transmitter and receiver configuration of the three, seems to be capable enough at shorter transmission distances than 1000 km regardless of fiber type. Carrier-suppressed return-to-zero (RZ) format, being the most tolerant to the self-phase modulation effect, showed better performance with fibers having larger chromatic dispersion. However, its transmission distance with low dispersion fibers is severely limited by the four-wave mixing effect. Bit-synchronous intensity modulated differential phase shift keying (IM-DPSK) format seems to be the best choice for a transmission distance beyond 1000 km because of its superior tolerance to optical noise and fiber nonlinear effects regardless of fiber types, despite slightly more complex transmitter and receiver configurations.

Compact polarization-independent optical circulator.

This paper introduces a compact polarization-independent optical circulator for communications using multimode optical fibers. This circulator consists of two pairs of rutile prisms used as polarization separators, a YIG Faraday rotator, a ring-shaped magnet, and a quartz halfwave plate. Insertion losses of 2.3 dB including fiber connection losses and isolations of 32 dB at a wavelength of 1.3 epsilonm were obtained using this circulator, and stabilization of the semiconductor laser was confirmed. Details on the design, fabrication, and characteristics of this circulator are presented.

Optical heterodyne image-rejection receiver for high-density optical frequency division multiplexing system

An optical heterodyne image-rejection receiver (IRR) for high-density optical frequency division multiplexing (OFDM) systems is described. The IRR was realized using balanced receivers, which showed more than 18-dB suppression over the 1.5-3.0-GHz IF region. Measured crosstalk penalties in a two-channel 560 Mb/s differential phase-shift keyed (DPSK) heterodyne optical communication system were realized for the first time. The crosstalk penalties in an OFDM system are estimated theoretically with and without the IRR. The required channel spacing and number of channels that can be accommodated in the 10-nm tuning range of the local laser are presented. A particular configuration of the IRR, its operation, and its performance limitations are discussed. The experimental results for image-rejection reception in a two-channel 560-Mb/s DPSK system are also given. Crosstalk penalties are estimated experimentally and compared to the theoretical calculation. Since the conventional configurations of the IRR are very sensitive to the polarization fluctuation of the transmitted signals, polarization-insensitive IRRs are proposed and their features are considered. >

Optical coherent broad-band transmission for long-haul and distribution systems using subcarrier multiplexing

The system configuration and future possibility of a coherent optical transmission system using subcarrier multiplex- ing (SCM) techniques are described. First, signal multiplexing techniques for coherent optical trans- mission are compared, and appropriate application for coherent SCM system is discussed. By comparing optical modulation meth- ods, optical frequency modulation (FM) using direct modulation of a DFB-LD and a heterodyne detection is shown to be feasible. lkansmission system using a configuration with a local laser in the transmitter is unaffected by polarization and is cost-effective. Phase noise can be suppressed by a phase-noise-canceling circuit (PNC) in a heterodyne receiver. This circuit is also effective for compensating for the frequency instability of light sources. A theoretical simulation of a coherent SCM system showed that a 100 channel of 30-MHz FM signal or a 15 channel of 155-Mb/s signal can be distributed to 10 000 subscribers using single stage or double stage optical amplifiers. To confirm the feasibility of coherent SCM transmission, pre- liminary experiments have been performed with optical FM by 2-channels 560-Mb/s ASK-SCM signal and a heterodyne detection using 3-electrode DFB-LDs. With a local LD setting at the transmitter, a span-loss margin of 40.7 dB was obtained using single stage Er3+-doped fiber amplifier (EDFA), and the system was insensitive to the state of polarization in the transmission fiber. In a subcarrier transmission of 560 Mb/s DPSK signal, the PNC effectively suppressed phase noise and the influence of IF frequency deviation. Finally, application of coherent SCM combined with optical frequency division multiplexing (OFDM) is discussed. A model allocation of optical carriers is proposed and possible receiving channel number is estimated.

Reflected light in the coupling of semiconductor lasers with tapered hemispherical end fibers.

The amount of reflected light fed back in the coupling of semiconductor lasers and tapered hemispherical-end (TH) fiber is estimated. Reflection from incident hemisphere is estimated to be below −50 dB, from the loss in the coupling between the laser and its virtual image. For multimode TH fiber reflection of backward-propagating light from connections can be reduced to below −50 dB by an ∼10-μm lateral displacement parallel to the junction plane; there is a small penalty in forward-coupling efficiency. The reduction of reflection is confirmed by measuring relative intensity noise of coupled laser light.

Optical isolator for semiconductor lasers

An optical isolator for semiconductor lasers, consisting of a polarizer and a quarterwave plate, was investigated experimentally. It was assembled in a compact laser-to-fiber coupling module. Experiment showed forward loss of 1.15 dB, backward loss of 24.8 dB, and rotative tolerance of 2.4 deg for 1 dB down. The length of the module is 43.2 mm. The coupling efficiency to a graded-index multimode fiber is 37.5%. The extinction ratio is improved by 6.75 dB. The suppression effect on the influence of reflected light was investigated, using the assembled module.

Intensity noise of InGaAsP/InP lasers under the influence of reflection and modulation

Abstract The noise characteristics of a 1.3 μm InGaAsP/InP laser diode were measured together with spectrum characteristics applying modulation and wide range quantitative reflection for the first time. The portion of reflected light which is fed back to the active region by a near mirror is estimated by simple coupling formula assuming gaussian beams. The noise characteristics, including sensitivity to reflection and enhancement or suppression effect by modulation, are discussed.