GB2226127A

GB2226127A - Optical frequency monitor

Info

Publication number: GB2226127A
Application number: GB8829333A
Authority: GB
Inventors: Jonathan Paul King
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1988-12-16
Filing date: 1988-12-16
Publication date: 1990-06-20
Also published as: GB8829333D0; GB2226127B

Abstract

An optical frequency monitoring arrangement including high-pass and low-pass optical filter means (21, 22), means for feeding (20) an optical signal to both filter means and means for determining the ratio of the transmitted optical powers through the filters. The arrangement may be used in a feedback loop to provide frequency control of a laser light source in an optical communication system, Fig. 5 (not shown). <IMAGE>

Description

OPTICAL FREQUENCY MONITOR.

This invention relates to an optical frequency monitoring arrangement for use in optical transmission systems.

In a single channel optical transmission system channel identification is not needed for optical frequency control. However, with multiple source inputs, some form of channel identification is required. Channel identity can be ascertained using an optical frequency monitor to determine the optical frequency of each source.

According to the present invention there is provided an optical frequency monitoring arrangement including an optical filter means having a prescribed frequency transmission characteristic, means for feeding an optical signal to the filter means and means for determining the ratio of the optical power transmitted by the filter means relative to the optical power of the signal at other frequencies.

According to one aspect of the invention there is provided an optical frequency monitoring arrangement including high-pass and low-pass optical filter means, neans for feeding an optical signal to both filter means and weans for determining the ratio of the transmitted optical powers through the filters.

The two filters have transmission characteristics which vary with increasing optical frequency (over the required monitoring range) from high-to-low and low-to-high respectively.

For any given optical frequency inside the operating range of the arrangement there is a unique ratio of the transmitted powers through each filter. The optical frequency of a source can be determined by measurement of the ratio of the optical powers passed by the high and low pass filters. The ratioing function provides an optical frequency measurement which is unaffected by any uncertainty of, or change in, the mean output power of the source.

The measured power ratio can be compared with a reference ratio (set to determine the desired frequency) to derive a control signal for the optical frequency control system of each source.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 illustrates graphically how the change in ratio between the outputs of high and low-pass filters represents a change in input frequency, Figure 2 illustrates schematically an implementation of an optical frequency monitor arrangement using separate high-pass and low-pass filters, Figure 3 illustrates schematically an alternative implementation of an optical frequency monitor arrangement using a wavelength dependent optical fibre coupler, Figure 4 illustrates graphically typical coupling ratios versus input optical frequency for a wavelength dependent optical fibre coupler1 Figure 5 illustrates schematically a single source optical frequency control arrangement, and Figure 6 illustrates schematically a multiple source optical frequency control arrangement.

A characteristic of optical filters is that the transmission varies with increasing (or decreasing) optical frequency. Thus as the frequency increases the transmission of a high-pass filter will increase, as shown in Figure 1. Likewise as the frequency increases the transmission of a low-pass filter will decrease, and vice versa.

Therefore when a varying frequency optical signal is applied to a high-pass filter and a low-pass filter simultaneously the ratio between the transmitted powers will change. If the transmitted powers are measured over the range to be monitored the ratio can be used as a measure of the input frequency. Thus, for optical frequency fll there will be a ratio *h1/*l1 which is different from the ratio *h2/*12 for frequency f21 where * is optical wavelength.

The optical frequency monitor arrangement of Figure 2 comprises a wavelength insensitive optical coupler 20, e.g. a fused fibre optical coupler, to one input port of which the optical signal is applied. The coupler is designed to provide substantially equal outputs to two optical filters, namely a high-pass optical filter 21 and a low-pass filter 22. Each filter is followed by a photodetector 23, 24 with a respective amplifier. The electrical outputs are then fed to a monitor and control circuit (not shown) where the ratio of the transmitted optical powers is determined and the requsite control signal derived.

In the alternative arrangement shown in Figure 3 a wavelength dependent optical fibre coupler 30 is used. Such a device is disclosed in our British patent No. 2150703B. At the low frequency end of the monitoring range substantially all the optical power input to the coupler is passed to the output fibre 31. As the optical frequency rises there is an increasing proportion of the input power transferred to output fibre 32 until at the highest monitoring frequency substantially all the power is delivered at fibre 32. Thus the coupler 30 forms in effect both a high-pass filter and a low-pass filter in the same device. Again two photodetectors 33, 34 with attendant amplifiers feed the fibre outputs to the control circuitry (not shown). A typical wavelength versus coupling ratio for a wavelength dependent coupler is shown in Figure 4. The coupling characteristic is essentially periodic.However, in many optical systems the range of optical frequencies will be limited by other factors to a defined optical frequency band. The coupler characteristics can be tailored to suit an arrangement specific to this range of inputs. The optical frequency discrimination, and the range over which monitoring can take place can be controlled during the manufacture of the coupler. In general, the longer the effective coupling length of the coupler, the steeper the coupling ratio vs wavelength characteristic and the shorter the unambiguous operating range of the arrangement will be. For example, a direct detection wavelength division multiplexed system based on distributed feedback (DFB) lasers might operate over a 100 nm band around 1500 nm.Any single DFB laser has a limited variation of output optical frequency (depending largely on its temperature and drive current), certainly less than + 10 nm. Thus the operating range of an optical frequency monitor would need to be 120 nm around 1500 nm for unambiguous optical frequency monitoring of this system. Ideally the coupler would have a 3dB coupling ratio at the mid-point of the spectral range over which it is desired to monitor frequency. However, in practice, providing the coupling vs wavelength characteristic has constantly either positive or negative slope over the desired monitoring range, no ambiguities arise.

Figure 5 shows an arrangement for providing frequency control for a single source. A source laser 50 launches the signals to be transmitted into an optical fibre 51. A tap 52 takes a small amount of the transmitted power and feeds it to an optical frequency monitor 53, e.g. such as that shown in Figure 3. The outputs of the monitor are compared in comparator 54 and the result is compared with a reference level in operational amplifier 55. The output of amplifier 55 forms a control signal which is fed to a frequency control circuit 56. Control circuit 56 develops a control voltage which is applied to the laser drive circuit 57 and controls bias current, thereby controlling the laser frequency . The control circuit 56 can also provide a signal to a temperature compensation circuit 58 to make laser frequency changes through use of the temperature tuning effect.

If more than one channel is to be monitored, as -is the case for multi-channel systems, some form of channel identification is necessary. This is so that the power transmitted by each optical filter for each individual channel is distinguishable from the contributions of the other channels.

For example, one means of channel identification in a multi-channel system could be to assign a unique sub-carrier modulation frequency, W(n) to each channel Tx The sub carrier modlilat on could be introduced by modulation of laser bias current, to produce a low modulation depth amplitude ripple on the mean output power of the source.

Thus for a multiple channel system the same basic optical frequency monitor 60 can be used for all the channels, as shown in Figure 6. For each of the channels 1, 2,...n a fraction of the output power is tapped off and all the tapped signals are fed to the common monitor 60. The two Outputs, high-pass and low-pass, are applied to each channel transmitter control circuit Tx(l), Tx(2)...Tx(n). In each control circuit the high and low-pass filter signals ratio is taken at the individual channels characteristic sub-carrier modulation frequency, W(1)1 W(2)),....W(n), by by passing the signals through respective band-pass filters Hf, Lf etc to a comparator and then to an optical frequency control circuit similar to that of Figure 5. The reference levels REF (n) at the comparators determine the optical frequency to which each transmitter controls. Thus the multiple channels can be monitored in parallel (other than sequentially).

Claims

CLAIMS.

1. An optical frequency monitori arrangement including an optical filter means having a prescribed frequency transmission characteristic, means for feeding an optical signal to the filter means and means for determining the ratio of the optical power transmitted by the filter means relative to the optical power of the signal at other frequencies.l.

2. An optical frequency monitoring arrangement including high-pass and low-pass optical filter means, means for feeding an optical signal to both filter and means for determining the ratio of the transmitted optical powers through the filters.

3. An optical frequency monitoring arrangement according to claim 2 wherein said means for feeding the optical signal comprises a wavelength insensitive fused fibre optical coupler.

4. An optical frequency monitoring arrangement according to claim 2 wherein said means for feeding the optical signal and said high and low-pass filtering means are together comprised of a wavelength dependent fused fibre optical coupler.

5. An optical frequency control arrangement for a laser including a frequency monitoring arrangement according to claim 2, 3 or 4, the control arrangement further including means for providing the monitoring arrangement with a fraction of the laser output, means for comparing the output of the monitor arrangement with a reference signal, means for deriving a feedback control signal from the output of the comparing means, and means for applying the control signal to control the optical frequency of the laser.

6. An optical frequency monitoring arrangement substantially as described with reference to the accompanying drawings.

7. A method of monitoring the frequency of an optical signal including the steps of high-pass filtering a fraction of the signal, low-pass filtering a fraction of the signal, and measuring the rates of the powers of the high-pass and low-pass filtered fractions of the signals.