We develop and demonstrate a technique for implementing a wide-band optical single-sideband suppressed-carrier modulator. The technique relies on a dual-electrode traveling wave modulator for sideband suppression and a Sagnac fiber-loop interferometer for carrier suppression. The implementation uses all commercially available components and demonstrates >40 dB optical carrier suppression, and as much as 60 dB unwanted signal sideband rejection across a 6-20 GHz microwave frequency range. The modulator is used to demonstrate broad-band spectral shifting- and inversion across the 3-18 GHz frequency range.

Optical single-sideband suppressed-carrier modulator for wide-band signal processing

An optical transmission system utilising at a transmitter (100) continuous phase frequency shift keyed modulation of a laser (14) and a coherent heterodyne receiver (200) therefor, the system including at the transmitter means for imposing (18, 12) a low frequency modulation on the transmitter optical power and frequency deviation and at the receiver means for extracting (28) from the receiver AGC control signal a replica of the low frequency modulation, means for demodulating (32) the modulated frequency deviation of the received optical signal synchronously with respect to the extracted low frequency signal, means for deriving (34) from the demodulated signal an error signal and a control system responsive to the error signal to control the receiver local oscillator (22) frequency.

Optical transmission system

A system for stabilizing a frequency separation between two coherent light sources includes first and second current sources driving first and second semiconductor laser devices with first and second frequencies, respectively. Light from the semiconductor laser devices is combined in a multiplexer, and a portion the combined light is diverted to an optical-to-electrical transducer. Output current from the transducer is combined with an output of a local oscillator driven at a selected frequency. A combined signal is then filtered, and used to form a control signal of one of the semiconductor laser devices.

Frequency separation stabilization method for optical heterodyne or optical homodyne communication

In an external-cavity tunable wavelength light source using a semiconductor laser (LD) (1), when an interferometer serving as an optical filter is constituted by part of an external cavity, an external-cavity longitudinal mode can be selected with good reproducibility, and a side mode suppression ratio can be increased. A light beam emitted from an AR-coated facet of an LD (1) is converted into a collimated beam by a lens (2), and the collimated beam is incident on a Mach-Zehnder (MZ) interferometer (5) serving as an optical filter. An interference light beam emerges from the MZ interferometer (5) toward a diffraction grating (3) serving as a preselector. Only a light component having a specific wavelength and selected by spectrally dispersing the interference light beam reversely travels in the same optical path as that of the interference light beam and is returned to the LD (1). In this manner, a cavity is formed between the LD (1) and the diffraction grating (3), and a laser beam having a wavelength determined by three relationships between an external-cavity longitudinal mode, a cavity loss caused by the diffraction grating (3), and the output characteristics of an interference light beam from the MZ interferometer (5) is oscillated.

Tunable Wavelength light source incorporated optical filter using interferometer into external cavity

A telecommunications system provides telephone services to subscriber locations over dedicated optical fibers which extend in a “star” configuration from a telephone central office. A transmultiplexer installed at the central office is connected between a digital switch and the optical fibers. The transmultiplexer has a plurality of FDM ports, each of which is connected to a respective one of the optical fibers. The transmultiplexer is connected by one or more TDM ports to the digital switch. Preferably, the transmultiplexer provides a concentrating function whereby the number of subscriber lines served by the transmultiplexer is greater than the total voice-grade channel capacity of the connection between the transmultiplexer and the digital switch.

Telecommunications system including transmultiplexer installed between digital switch and optical signal transmission fiber

A method and apparatus for locking the frequencies of a number of first beams of coherent radiation from respective sources. Typically, the sources will form local oscillators in respective transmitters. The apparatus includes a reference beam generator such as a HeNe laser which generates a primary reference beam fed to an interferometer which generates a second, reference beam composed of a number of reference frequencies. This second reference beam is caused to interfere with each of the beams from the local sources in an optical coupler and the resultant beam is detected with a photodiode. The electrical output signal of the photodiode is filtered and fed to a discriminator which controls the frequency of the local laser. The discriminator controls the laser to maintain the relationship between the frequencies of the laser and the reference laser substantially constant.

Frequency locking radiation beams

The operating principles and performance of waveguide Mach-Zehnder frequency translators suitable for use in coherent optical fibre systems are described. The performance of a coherent optical fibre system transmitting analogue colour video signals and employing a Mach-Zehnder electro-optic waveguide device as a double sideband suppressed carrier frequency modulator is reported. It is shown that the receiver sensitivity of such a system may be as much as 21 dB greater than that for an equivalent direct detection system. The use of electro-optic frequency translators in coherent optical networks to assemble a frequency multiplex of wideband signals is described. Particular attention is given to the linearity of devices, in systems employing up to four channels, and it is shown that unwanted products can be suppressed to 30 dB below the signal level. The use of this type of device as a frequency control element in optical multichannel networks is also considered.

Electro-Optic Frequency Translators And Their Applications In Coherent Optical Fibre Systems

Apparatus for converting a frequency division multiplex (fdm) signal to a time domain multiplex (tdm) signal in which a voltage control oscillator whose output is a staircase of reference frequencies is mixed by a mixer with a received fdm signal. Each received frequency is output in turn at a common intermediate frequency from a bandpass filter thereby forming a tdm signal after passing through a demodulator and low pass filter. The conversion may take place in the optical domain by mixing optical reference frequencies with an optical fdm signal in, for example, a coherent heterodyne optical receiver.

Godfrey Richard Hill

Papers

Frequency locking radiation beams

Electro-Optic Frequency Translators And Their Applications In Coherent Optical Fibre Systems

Apparatus and method for converting a frequency division multiplex to a time division multiplex