WO1993016514A1

WO1993016514A1 - Frequency changing of light signals

Info

Publication number: WO1993016514A1
Application number: PCT/IE1993/000004
Authority: WO
Inventors: Gerald Farrell; Paul James Aquinas Phelan; John Hegarty
Original assignee: Optronics Ireland; The Provost, Fellows And Scholars Of Trinity College
Priority date: 1992-02-14
Filing date: 1993-02-12
Publication date: 1993-08-19
Also published as: AU3462093A

Abstract

A source light signal (6) is inputted to a self-pulsating laser diode (2). A frequency (fundamental or harmonic) of the diode (2) is matched with a frequency (fundamental or harmonic) of the source light signal (6). The resulting output light signal (7) from the laser diode (2) is at the fundamental frequency of the self-pulsation. Accordingly, depending on the choice of matching of the harmonic and fundamental frequencies of the source light signal and the laser diode, synchronised frequency division or multiplication can occur.

Description

"Frequency chanσinσ of light signals"

The invention relates to the changing of the frequency of signals carried by light.

At present, frequency changing of a light or optical signal is usually carried out by reception of the light signal, converting it to an electronic format and changing the frequency electronically. An example is the de¬ multiplexing of an optical signal, where it is initially converted to an electronic signal which is subsequently de-multiplexed using electronic circuits.

In the paper "A 4 x 5 Gb/s Transmission System with All- Optical Clock Recovery", by P.E. Barnsley, G.E. Wickens, H.J. Wickes, and D.M. Spirit in IEEE Photonics Technology Letters Vol. 4, No. 1, pp 83-86, 1992. In this paper, clock extraction is performed after demultiplexing. This is clearly unsatisfactory because electronic processing is increasingly proving to be a limitation in the development of higher signal speeds. Another disadvantage is that conversion to electronic format for processing leads to increased complexity.

It is an object of the invention to provide a method and apparatus for changing the signal frequency of light signals without the need for conversion to electronic format. Another object is that the method and apparatus be simple and inexpensive.

According to the invention, there is provided a method of changing frequency of a modulated source light signal, the method comprising the steps of:-

matching the fundamental or an harmonic frequency of self-pulsation of a self- pulsating laser device with the fundamental or an harmonic frequency of the source light signal; and

inputting the source light signal to the laser device to cause output of a light signal having a modulated frequency substantially equal to the fundamental frequency of the laser device and being in synchronism with the source light signal.

In one embodiment, the optical wavelength of the laser device and of the source light signal are initially matched.

In another embodiment, an harmonic frequency of the device matches a frequency of the source light signal for frequency division. In this latter embodiment, the fundamental frequency of the source light signal may be matched for frequency division by an integer.

In a further embodiment, an harmonic frequency of the source light signal matches a frequency of the device for frequency multiplication. In this latter embodiment, the fundamental frequency of the device may be matched for multiplication by an integer.

For frequency multiplication, the source light signal is preferably pulsed.

In one embodiment, the laser device is a laser diode, and preferably a multi-section laser diode such as a twin- section laser diode. In another embodiment, the source light signal is a time division multiplexed signal and the output signal is a synchronised clock of the multiplexed signal.

According to another aspect, the invention provides an apparatus for frequency changing of a modulated source light signal, the apparatus comprising:-

a laser device capable of self-pulsation;

means for operating the laser device in the self-pulsation mode;

means for matching the fundamental or an harmonic frequency of the device with the fundamental or an harmonic frequency of the light signal;

means for inputting the source light signal to the laser device to cause output of a light signal having a modulated frequency substantially equal to the self-pulsating fundamental frequency of the laser device and being in synchronism with the source light signal.

In one embodiment, the laser device is a multi-section semiconductor laser diode having an absorber contact. The diode may preferably comprise an absorber control circuit for self-pulsating frequency control.

In another embodiment, the absorber control circuit comprises a voltage regulator having a series pass transistor output and a resistive load connected in parallel. The invention will be more clearly understood from the following description of some preferred embodiments thereof, given by way of example only with reference to the accompanying drawings in which:-

Fig. 1(a) is a diagram showing an experimental frequency changing apparatus of the invention and Fig. 1(b) is a circuit diagram showing an absorber circuit of the apparatus in detail;

Fig. 2 is a table showing the manner in which frequency may be changed using the apparatus; and

Figs. 3 to 6 inclusive are graphs showing the manner in which frequency is changed by the method and apparatus of the invention.

Referring to the drawings and initially to Fig. 1(a), an apparatus 1 of the invention for changing frequency of a source light signal is illustrated. The apparatus 1 comprises a multi-section laser diode, in this embodiment a twin-section laser diode 2 having an active region 3, a forward bias contact 4 and an absorber contact 5. Any type of self-pulsating laser device may be used instead of a multi-section laser diode. The important point is that it is capable of self-pulsation without an externally applied modulation.

A source or input light signal 6 is shown being inputted to the active region of the diode 2. The apparatus 1 also comprises an absorber control circuit 8 connected to the absorber contact 5 of the diode 2 via a bias T 8(a), which is in turn connected to a resonant transmission line stub 8(b). The source light signal 6 may be generated in any suitable manner, and indeed in practice it may be received from a remote source. In the following description, the light signal 6 is generated by a grating external cavity (GEC) laser 10. This is an experimental set-up which is suitable for illustrating the invention.

The GEC laser 10 has a laser diode 11 in which one end of the active region has been coated so that reflection is suppressed. The GEC laser 10 also comprises lenses 12 mounted at each end of the diode 11 and a rotatable grating 13. The grating 13 may be rotated very precisely and is used to provide optical feedback at a single wavelength, where the position of the grating 13 decides the wavelength. Thus, the GEC laser 10 is tuneable over a range of approximately 30 nm. An isolator 14 is mounted in line with a lens 12, and there is a further lens 15 beyond the isolator 14. A bias circuit 16 is connected to the GEC laser 10, and this is in turn connected to a frequency synthesizer 17 connected to modulate the DC drive current applied to the GEC laser 10. This in turn modulates the optical output of the GEC laser 10. The lens 15 at the output of the isolator 14 is used to couple the light signal into a single mode optical fibre. This fibre passes through a polarisation controller 18, the output of which is the source light signal 6. An optical fibre splitter 19 is connected to a power meter 20. The source light signal 6 is injected into the twin-section laser diode 2 using a lens-ended fibre.

The output light signal 7 is coupled into a multimode fibre connected to an avalanche photodiode 21 which is used to observe the signal on both a spectrum analyzer 22 and a high speed oscilloscope 23. Referring now to Fig. 1(b), the absorber control circuit 8 is shown in more detail. The circuit 8 comprises a voltage regulator 30 with a feedback control circuit 31 and a series pass transistor output 32 having a resistive load 33 in parallel. The voltage regulator 30 sets the forward voltage operating point for the absorber contact 5. Reverse optically generated current from the circuit 8 flows only in the resistive load 33 and does not interfere with the operation of the regulator 30.

The apparatus 1 may be used in several different manners to change signal frequency of the input light signal 6, which in this embodiment is amplitude modulated. By control of the laser diode 2, or of the GEC laser 10, the fundamental or an harmonic RF frequency of the source light signal 6 is chosen to match the fundamental or an harmonic frequency of self-pulsation of the laser diode. In one example, the frequency synthesizer 17 and the bias circuit 16 are used to control the GEC laser 10 to amplitude modulate the input light signal 6 to a desired modulated frequency.

If, for example, the amplitude modulated frequency matches that of an harmonic of the self-pulsating laser diode 2, synchronous frequency division occurs because the output light signal 7 has a frequency substantially equal to that of the fundamental self-pulsation frequency of the laser diode 2. The fact that the self-pulsating laser diode 2 has strong harmonics at multiples of its fundamental frequency is important in achieving this. What happens is that the input light signal synchronises through this overlap causing the self-pulsating frequency to change. Of course, if the fundamental frequencies of the source light signal 6 and of the diode 2 match, then synchronisation alone, without frequency changing, occurs. This locking is improved if the operating optical wavelength of the input light signal 6 coincides with the operating wavelength of the laser diode 2. This may be achieved by setting the operating wavelength of the GEC laser 10 or alternatively by temperature control of the laser diode 2. In this case, the operating wavelength is centred on 1616 n . It has been found that the operating wavelength of the input light signal must be less than or equal to the operating wavelength of the laser diode for locking to occur.

Referring now to Fig. 2, representative samples of frequency changing are illustrated. For illustrative purposes, the figures given are approximate. The input signal 6 is sinusoidal and has a fundamental frequency of 900 MHz. The absorber control circuit 8 is adjusted so that there is a self-pulsating fundamental frequency of 450 MHz and the input signal frequency matches the first harmonic frequency of the diode 2 and frequency division by 2 occurs. The frequency matching is represented by the arrowed line in Fig. 2. Referring to Fig. 3, frequency division of input signals b and d are shown for frequency division by 2 and frequency division by 3, respectively.

Referring now to the remainder of Fig. 2 and to the graphs of Figs. 4 to 6, the manner by which frequency multiplication is achieved is described. The GEC laser 10 is actively mode locked by tuning the frequency synthesizer 17 to exactly 539.578 MHz. This is the cavity round trip frequency of the GEC laser 10. When the DC bias to the GEC laser 10 is reduced to 18.6 mA, and the RF power is increased to +22 dBm, single clean pulses are obtained with a duration of less than or equal to 200 ps. The absorber control circuit 8 is adjusted so that the self-pulsating fundamental frequency of the laser diode 2, without injection of the input signal, closely matches the second harmonic of the input signal 6. The resonant transmission line stub 8(b) improves the frequency changing operation of^'the diode 2 by reducing the RF noise of the diode. This results in a reduction of the optical power required for synchronisation and an improved quality of synchronisation for a fixed optical input power.

When the 540 MHz mode locked light signal 6 is injected into the diode 2, the self-pulsating fundamental frequency (1080 MHz) locks on to the first harmonic of the input signal 6, at 1080 MHz. This is shown by the arrowed line in Fig. 2. Thus, the output signal which has a frequency substantially equal to the self-pulsation fundamental frequency of the diode 2, namely 1080 MHz, represents frequency multiplication by 2 from 540 MHz. This is shown in Fig. 4, in which the lower trace is the pulsed input signal 6 at about 540 MHz and the upper trace is the self- pulsation output at a frequency of 1080 MHz. An important point for frequency multiplication is that an input signal having strong harmonics is used. One suitable way of achieving this is when the source signal is pulsed. Needless to say, frequency multiplication by integers other than two is also possible, depending on the selection of self-pulsation fundamental frequency of the laser diode 2. For frequency multiplication by 3, this may be chosen to match the second harmonic frequency of the input signal.

Referring now to the third example in Fig. 2 and to Fig. 5, frequency multiplication by a non-integer number is illustrated. The frequency of the input signal 6 is set at 427.5 MHz and this is injected into the laser diode 2 which is self-pulsating at a frequency close to 640 MHz. As shown in Fig. 2, the self-pulsating first harmonic of the diode 2 is 1280 MHz, which is close to the second harmonic of the input signal 6, namely, 1282.5 MHz. Synchronisation then takes place if sufficient power is contained in the overlapping harmonics. Thus the output signal of 640 MHz is 1.5 times the input signal frequency of 427 Mhz. These signals are shown in Fig. 5 in which the upper trace is the input signal 6 mode locked at 427.5 MHz, and the lower trace is the output signal 7 which is at a frequency of 641.25 MHz. This time it will be seen that where an harmonic frequency of the input signal matches an harmonic frequency of the self-pulsating laser diode frequency multiplication by a non-integer can be achieved.

The method of the invention may be applied to a very wide range of practical applications. One example is in de¬ multiplexing in a time division multiplexed optical communication system. In this case the output signal would be a synchronised clock signal of the multiplexed signal where frequency division is used.

Another application is in the field of optical frequency synthesizers. For example, a long external cavity laser may be used to provide a master signal which is rich in harmonics in the frequency range over which self-pulsation takes place. This master signal is used to lock the self- pulsating laser diode. By altering only the absorber DC bias, it is possible to produce one of a range of stable output frequencies from the twin section laser diode. For example, the GEC laser 10 may be driven to produce a master signal at the fundamental frequency of 427.9 MHz, with an injection power into the laser diode 2 of 7.2 uW. For absorber voltages of 0.293 V, 0.335 V and 0.368 V, the corresponding output frequencies are 427.9 MHz, 855.9 MHz, and 1283.7 MHz, respectively. The actual self-pulsing output waveforms a, b, and c are shown in Fig. 6, for increasing absorber voltage and increasing (synthesised) frequency. The advantages of this technique include the facts that outputs are locked to a stable, low jitter master source, and DC tuning only is required to produce a range of outputs.

Another application is optical clock distribution. In this case a master clock (for example from a mode-locked laser) is sent to a number of self-pulsing lasers. Each self-pulsing laser may, by DC control of the self-pulsing frequency, synchronise to the master clock but "change" the master frequency to another frequency. In this way, clocks at many frequencies may be distributed to different parts of the system, (for example a computer system or a telephone exchange system) with the accuracy of the master clock.

Other practical applications will be readily apparent to those skilled in the art, these being based on the synchronisation and frequency changing features of the invention.

The invention is not limited to the embodiments hereinbefore described. It will be appreciated that the apparatus described is experimental and of course in practice control of the source light signal may not be possible in many instances. In this case, it will be necessary to change the frequency of self-pulsation of the laser diode to match the relevant frequency of the source light signal.

An important aspect of the invention is that a self- pulsating laser device is used. In the embodiments described, a twin-section laser diode is used, however, it is envisaged that, if a diode is used, it may be of the multi-section type having three or more sections. Alternatively, other types of self-pulsing lasers such as self-pulsing DFB lasers, polarisation switching lasers, or indeed self-pulsing lasers as used in optical disk systems will be capable of performing the same functions. Further, the invention is not limited to frequency changing of intensity modulated light signals as frequency modulated light signals could alternatively be used.

Claims

A method of changing frequency of a modulated source light signal, the method comprising the steps of:-

A method as claimed in claim 1 wherein the optical wavelength of the laser device and of the source light signal are initially matched.

A method as claimed in claims 1 or 2, wherein an harmonic frequency of the device matches a frequency of the source light signal for frequency division.

A method as claimed in claim 3 wherein the fundamental frequency of the source light signal is matched for frequency division by an integer.

A method as claimed in claims 1 or 2 wherein an harmonic frequency of the source light signal matches a frequency of the device for frequency multiplication.

6. A method as claimed in claim 5 wherein the fundamental frequency of the device is matched for multiplication by an integer.

7. A method as claimed in claims 5 or 6 wherein the source light signal is pulsed.

8. A method as claimed in any preceding claim wherein the laser device is a laser diode.

9. A method as claimed in claim 8, wherein the laser diode is a multi-section laser diode.

10. A method as claimed in claim 9 wherein the multi- section laser diode is a twin-section laser diode.

11. A method as claimed in any of claims 1 to 4, wherein the source light signal is a time division multiplexed signal and the output signal is a synchronised clock of the multiplexed signal.

12. An apparatus for frequency changing of a modulated source light signal, the apparatus comprising:-

a laser device capable of self-pulsation;

_ means for operating the laser device in the self-pulsation mode;

means for matching the fundamental or an harmonic frequency of the device with the fundamental or an harmonic frequency of the light signal; and means for inputting the source light signal to the laser device to cause output of a light signal having a modulated frequency substantially equal to the self-pulsating fundamental frequency of the laser device and being in synchronism with the source light signal.

13. An apparatus as claimed in claim 12, wherein the laser device is a multi-section semiconductor laser diode having an absorber contact.

14. An apparatus as claimed in claims 12 or 13, further comprising an absorber control circuit for self-pulsating frequency control.

15. An apparatus as claimed in claim 14 wherein the absorber control circuit comprises a voltage regulator having a series pass transistor output and a resistive load connected in parallel.