Yoshihito Onoda

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. >

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.

An optical isolator for semiconductor lasers in the 0.8 μm range

Abstract To reduce the instability of laser operation due to reflected light an isolator for semiconductors lasers in the 0.8 μm range is developed using a paramagnetic Faraday rotation glass and multiple reflection technique. A closed loop magnetic circuit is adopted to surpress changes in backward characteristics due to accessions of external magnetic substance. A new reflection type polarizer is used. Forward loss of 0.5 dB and backward loss of 28 dB are obtained. The isolator is assembled in a laser-fiber coupling module and some effects of isolation are confirmed.