Abstract: In known coherent optical receivers comprising two detectors, optical 90.degree. hybrids have a `throughput` .ltoreq.25%. Theoretical analysis reveals that a higher `throughput` of up to a maximum of approximately 29.3% is possible. The invention provides such a 90.degree. hybrid (30) of the coupler type with the aid of a 2.times.2 port which is based on a loss-free 3.times.3 directional coupler having the maximum throughput ratios between two of the inputs (31/39, 32/40) and two of the outputs (39/34, 40/35) and the 90.degree. phase difference between said two outputs, the third input and the third output not being used. An integrated version employing indium phosphide is described.

Abstract: In known coherent optical receivers with two detectors, optical 90.degree. hydrids have a throughput .ltoreq.25%. Theoretical analysis shows that a higher throughput, up to a maximum of approximately 29.3%, is possible. The invention provides such a 90.degree. hybrid (30) of the polarisation type. In an optical hybrid (3) having a structure known per se, i.e. polarisation controllers (38, 39) at the inputs (40 and 41) of a power coupler (42), and polarisers (45, 46) at the outputs (43 and 44) thereof, the polarisation controllers and the polarisers are put into such positions, that the points on the Poincare sphere, which correspond to those positions, comply with two conditions which guarantee a 90.degree. phase difference and the higher throughput. For example, the positions correspond to points which represent positions in which the polarisation controllers produce mutually opposite elliptical polarisations at an angle .alpha. and the polarisers are placed linearly at angles .alpha. and -.alpha., and .

Abstract: An optical mixing device having intensity-noise suppression, for use in a heterodyne receiver, comprises a coupler (31) having two optical input ports (32, 33) for a received signal (E.sub.S) and a local oscillator signal (E.sub.L), respectively, and two optical output ports (34, 35). An optical path-length difference is created between light signals emerging from the two output ports (34, 35) and a photodetector (36), and the light signals are fed from the output ports (34, 35) to the photodetector (36) in separate beams. The mixing device has a simple optical fiber construction and does not require polarization-influence devices. A coherent heterodyne receiver is also provided which includes the optical mixing device.

Abstract: Optical circuit for a measuring system for measuring the reflection sensitivity of an optical transmission network, comprising a first and a second connection point (1, 4) for a first and a second optical signal source (S1, S2), respectively, a third connection point (2) for optical receiving means (R), optical reflection means (RF), and coupling means. The coupling means consist of a first waveguide junction (C2) via which the second connection point (4) is coupled in the forward signal direction to the reflection means, and a second waveguide junction (C1) via which the first connection point (1) is coupled in the forward signal direction to the third connection point. The two junctions are coupled in such a way that the reflection means are coupled in the forward signal direction to the third connection point via the first and second junctions.

Abstract: Optical reflective star device for transmitting signals between a plurality of transceivers, provided with a plurality of ports for distributing each signal supplied to one port over the other ports. The reflective star device is made up of a plurality of star elements having a plurality of reflection branches and a plurality of through-connections between the star elements. An optical amplifier is disposed in at least one of the reflection branches and/or through-connections.

Abstract: An optical transmitting and receiving system for connection to an optical bidirectional transmission path comprises a transmitter and a pair of coherent receivers sharing a local oscillator. The transmitter and receiver and connected to the transmission path via a hybrid circuit. The hybrid circuit is provided with a first polarisation splitter, a nonreciprocal polarisation rotator and a second polarisation splitter. A connection of the first polarisation splitter and a connection of the second polarisation splitter form, respectively, the input connection for the transmitter and the bidirectional connection of the hybrid circuit. The input and output of the nonreciprocal polarisation rotator are connected between the first and second polarisation splitters. The receivers are connected to separate outputs of the polarisation splitters.

Abstract: Frequency stabilizing in a, for instance star/branched-tree shaped, network can be well executed by locking the frequencies (f1) of the transmitters (T1) in the network center to a common reference frequency source (CFC) and, moreover, per duplex connection, locking (at a fixed frequency distance) the frequency (f2) of the subscriber's transmitter (T2) to the transmission signal (t1) as received from the network center's transmitter (T1). Per duplex connection only the subscriber's local oscillator (L2) has to be tuned so that the transmission signal from the network center is received optimally.

Abstract: Transmission system for the polarization-insensitive transmission of signals over a signal route (ST) between a first system connection point (S1) for a transmitter (T) and a second system connection point (S3) for a receiver (R), and between two further system connection points (S2 and S4) for an additional transmitter and a receiver. On one side the high-order retarder (HR) is disposed between the system connection points and the signal route (ST). The system comprises a first and a second hybrid circuit (H1, H2) each provided with two unidirectional connection points and a bidirectional connection point. The unidirectional connection points form the system connection points (S1 to S4 inclusive). The bidirectional connection point of one of the hybrid circuit (H1) is connected via the high-order retarder (HR), and the bidirectional connection point of the other hybrid circuit (H2) is connected directly, to the signal route (ST).

Abstract: A polarization-independent amplification device comprising an optical semiconductor amplifier. The amplification device input signal is fed to one side of the amplifier and a reflector is provided at the other side. The amplification device output signal, formed by the amplification device input signal reflected by the reflector and amplified by the amplifier, is taken off at the first side of the amplifier. The reflector is reciprocal, a signal or signal component fed thereto and having a polarization perpendicular to the principal axis of the amplifier being reflected as a reflection signal having a polarization which is virtually parallel to said principal axis and vice versa.

Abstract: The invention relates to a transmission system for the polarization insensitive transmission of signals over a signal path between a transmitter and a receiver, provided with high-order retarder. The retarder is a combination of a polarization-splitting coupling and a loop-type delaying transmission route. The retarder may be sited at the receiver end, with a suitably chosen angle between one of the principal axes of the retarder and the polarization of a local oscillator incorporated in the receiver. Advantages are reduction in size and number of components in the retarder, reduction in the number of retarders in such a transmission system and an increase of the capacity for integration.

Abstract: An optical reflective star device having a number of ports for distributing each signal fed to a port over the other ports. Said optical reflective star device is made up a number of star elements and comprises a number of reflectors provided at reflection points in the reflective star device. At least one of the reflectors is an orthogonal polarisation reflector.