Apparatus and Method for Detecting Mode-Hopping in Semiconductor Lasers

Abstract: An arrangement for discriminating whether a semi­ conductor laser (13, LD) employed in a light transmis­ sion system is functional or not includes: a circuit (11) for generating a pulse pattern (P1, Pn) repeatedly with unit of a plurality of bits and feeding to the semiconductor laser; a medium (3) for passing a light pulse signal output from the semiconductor laser; and a receiving equipment (2) including a circuit (21, APD) for regenerating the pulse pattern from the light pulse signal and outputting a regenerative pulse pattern (P2) and a discriminating circuit (26) responding to the regenerative pulse pattern. The discriminating circuit discriminates whether or not a pseudo-pulse exceeding a predetermined threshold level (V th ) appears at a specific position in the regenerative pulse pattern during a predetermined time interval and indicates a result of the discrimination. Based on the indicted result, a discrimination of whether the laser is functional or not can be carried out with simplicity and high reliability and without troublesome labor, under a condition that the laser is kept incorporated into the light transmission system.

Description: The present invention relates to detecting mode-hopping in semiconductor lasers. Such detection is useful, for example, in monitoring whether a semiconductor laser employed in transmitting equipment is functioning as intended. A semiconductor laser employed in a transmitting equipment of a light transmission system is modulated by current pulses of high bit rate such as 2 Gb/s and, usually, oscillated on a single wavelength. In such a case, the wavelength band of the emission spectrum may be shifted from the dominant mode to the next mode, for example, due to a change in temperature, or a change in level caused by a rise of the current pulse, or the like. This phenomenon is referred to as a "mode-hopping" or "mode jump". When mode-hopping occurs, an error arises in that, in the position at which a correct signal, e.g. data of "0", should appear, an incorrect signal, i.e. a pseudo-pulse of "1", appears due to a chromatic dispersion of an optical fibre. Although the error caused by the mode-hopping indeterminately occurs, a frequency of occurrence of the error, hereinafter referred to as an error rate, exceeding a certain maximum acceptable error rate is not preferably from the viewpoint of a realisation of a satisfactory transmission characteristic in the light transmission system. That is, the lower the error rate becomes, the better it is. In practical use, however, errors due to such mode-hopping rarely occur, and it is difficult to clarify the cause thereof. Where mode-hopping occurs in the semiconductor laser with a rate above the acceptable error rate, the measurement of the rate of the mode-hopping has heretofore been difficult and, further, it has been difficult to appropriately select the semiconductor laser by preliminary test procedures. Accordingly, the measurement of the error rate is carried out under a condition that the laser is kept incorporated into the light transmission system. In this type of measurement, however, even if an error is detected in the receiving system, it is impossible to exactly discriminate whether the error is caused by the semiconductor laser, or by a transmission line such as an optical fibre line. Namely, as stated above, it has been hard to make clear the cause of occurrence of the error. Therefore, where an error occurs with an error rate above the acceptable error rate in such a light transmission system, in order to find out whether the cause of the error is due to the mode-hopping of the laser, it becomes necessary to make an adjustment for all of the main elements constituting the whole system including the transmitting and receiving system for maintenance of the quality of the transmission characteristic to a constant level. This makes works or operations extremely troublesome or complicated, and thus is not preferable. An experimental arrangement capable of detecting mode-hopping of a semiconductor laser is described in "Stability of Two Modes in DFB Laser Diode under High Data Rate Modulation", Electronic Letters Vol. 20, No. 6, 15 March 1984. This arrangement may be considered to comprise a pattern generating circuit for generating repeatedly a plural-bit pulse pattern to be applied to a semiconductor laser under test so as to modulate its optical output; and an optical receiving circuit for employing such optical output, received by way of a transmission medium producing chromatic dispersion, to produce a regenerated pulse pattern. In this arrangement, however, mode-hopping is essentially detected by an optical discrimination technique using a spectrometer to select light pulses of one particular longitudinal mode of the laser. According to a first aspect of the present invention there is provided an arrangement for detecting mode-hopping of a semiconductor laser, which arrangement comprises: a pattern generating circuit for generating repeatedly a plural-bit pulse pattern to be applied to a semiconductor laser under test so as to modulate its optical output; and an optical receiving circuit for employing such optical output, received by way of a transmission medium producing chromatic dispersion, to produce a regenerated pulse pattern; characterised by a discriminating circuit connected for determining whether a pulse of an amplitude exceeding a predetermined threshold level appears at a preselected position in said regenerated pulse pattern, thereby to indicate whether mode-hopping is occurring in the semiconductor laser. Such an arrangement is capable of discriminating whether or not a semiconductor laser is functional with simplicity and high reliability, by detecting appearance of a sign of "1", i.e. a pseudo-pulse, at the preselected position in the regenerative pulse pattern (i.e. within a predetermined time interval), where a sign of "0" should normally appear. According to a second aspect of the present invention there is provided a method of detecting mode-hopping of a semiconductor laser, in which method a plural-bit pulse pattern is applied repeatedly to a semiconductor laser under test so as to modulate its optical output, and the optical output of the semiconductor laser, received by way of a transmission medium producing chromatic dispersion, is employed to produce a regenerated pulse pattern; characterised in that mode-hopping of the semiconductor laser is detected by determining whether a pulse of an amplitude exceeding a predetermined threshold level appears at a preselected position in the said regenerated pulse pattern. Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a diagram for explaining a problem in a communication system employing a semiconductor laser; Figure 2 is a block diagram of a system including an embodiment of the present invention; Figure 3 is a circuit diagram of a discriminating circuit shown in Figure 2; Figures 4a to 4g are waveform diagrams for explaining operation of the system shown in Figure 2; and Figures 5a to 5e are waveform diagrams for explaining a form of generation of a pulse pattern Pn employed in another embodiment of the present invention. Figure 1 schematically shows a diagram for explaining a problem in a previously-considered communication system, employing a semiconductor laser (not shown) in a transmitting system S. Assuming that a chain of current pulses for driving the semiconductor laser has an eight-bit formation, for example, "10000000". Where a data transmission is carried out correctly or exactly, a pulse signal having the same bit formation as the current pulses can or must be regenerated in the receiving system R. When the mode hopping occurs in the laser, however, an error arises in that, in the position at which a sign of "0" should originally appear (in the illustrated example, in the third pulse position), a pseudo-pulse, i.e. a sign of "1", appears. The position at which the pseudo-pulse appears is defined by a dispersion coefficient m of a medium such as an optical fiber F, a length L thereof, and a fluctuation Δλ from the dominant mode of the wavelength occurring due to the mode hopping of the laser. Although the error caused by the mode hopping occurs indeterminately, an error rate exceeding a set maximum acceptable error rate is preferably to be avoided to facilitate realization of a satisfactory transmission characteristic in the light transmission system. A high bit rate data transmission has recently been developed in the light transmission system and, to ensure a high quality transmission characteristic, it is demanded or recommended to treat as "error free" at an error rate of approximately 10⁻¹¹. Note, the error rate of 10⁻¹¹ means that, when the semiconductor laser is driven by 10¹¹ current pulses, a single error occurs. Nevertheless, an error in the system often occurs at the rate of 10⁻¹⁰, and the cause of occurrence thereof is difficult to make clear, as previously explained. In such a system, measurement of the error rate is carried out under the condition that the laser is kept incorporated into the light transmission system. This leads to difficulty in discriminating whether the error is due to the laser, due to the transmitting system, due to the receiving system, or due to the optical fiber F. Where an error occurs with an error rate of approximately 10⁻¹⁰ in such a light transmission system, the maintenance of the quality of the transmission characteristic to a predetermined level can be carried out by exchanging a laser diode of the semiconductor laser and adjusting the bias current thereof in the transmitting system, and by adjusting the bias voltage of an avalanche photo diode and changing a threshold level. After checking the whole system and confirming the whole system is good, it can be predicted that the reason of the error is the mode hopping of the laser. However, this makes works or operations extremely troublesome or complicated. And it is only prediction. A preferred embodiment of the present invention will now be described in detail with reference to Figs. 2 to 4g. Figure 2 illustrates a constitution of the arrangement, which discriminates whether a semiconductor laser is functional or not, as an embodiment of the present invention. The arrangement of Fig. 2 has a form of a semiconductor laser discrimination system employing the semiconductor laser which is similar to the transmitting system and is constituted by a transmitting equipment 1, a receiving equipment 2 and an optical fiber 3 connecting both of the equipments. The optical fiber 3 has a known dispersion coefficient m (ps/km/nm) and a predetermined length L (km). In the transmitting equipment 1, 11 denotes a pulse pattern generating circuit, which repeatedly generates a pulse pattern P1 of eight-bit formation with a bit rate of 2 Gb/s. The pulse pattern P1 is generated with an RZ ( r eturn to z ero) mode and composed of a sign of "1" of one bit and the subsequent signs of "0" of seven bits. The generated pulse pattern P1 is input to an amplifier 12, which amplifies the pulse pattern up to the level necessary for the oscillation of the subsequent semiconductor laser 13. The semiconductor laser 13 is constituted by, for example, a distributed feedback (DFB) laser. The laser 13 is modulated with an RZ mode by current pulses fed via the amplifier 12 and oscillates a laser beam with a single wavelength, i.e., a light pulse signal, which is transmitted via the optical fiber 3 to the receiving equipment 2. In the receiving equipment 2, 21 denotes a light receiving circuit having an avalanche photodiode (APD), which regenerates the pulse pattern P1 from the light pulse signal transmitted via the optical fiber 3. The regenerated or regenerative pulse pattern is indicated by reference P2. Reference 22 denotes an equalizer provided for compensating an attenuation characteristic in the transmission line, i.e., the optical fiber 3, and reshaping the waveform to obtain maximum signal to noise ratio. Reference 23 denotes a timing extraction circuit, which has a function of extracting a clock pulse P5 from the regenerative pulse pattern P2. The timing extraction circuit 23 includes a band-pass filter which passes only a specific frequency. Therefore, the clock pulse P5 has the same frequency as the bit rate of the transmitting data. Reference 24 denotes a 1/N frequency dividing circuit, which divides a frequency of the clock pulse P5 into one-eighth. The frequency divided clock pulse is indicated by reference P3. Reference 25 denotes a phase shifting circuit, which has a function of shifting the phase of the frequency divided clock pulse P3 by a predetermined phase difference  from the above position of the sign of "1". The phase shifted clock pulse is indicated by reference P4. In the present example, the phase difference  is selected to a time interval corresponding to a time width of each bit of the regenerative pulse pattern P2, or more. The phase shifted pulse pattern P4 is input, together with the pulse pattern P2 fed from the light receiving circuit 21, to a discriminating circuit 26, which will be stated later. Reference 27 denotes a counter effecting a counting operation based on a discrimination signal S3 fed from the discriminating circuit 26. Next, a constitution of the discriminating circuit 26 will be described with reference to Fig. 3. The illustrated circuit is constituted by an operational amplifier 31, an inverter 32 and an AND gate 33. The operational amplifier 31 compares a level of the regenerative pulse pattern P2 with a predetermined threshold level V th , and outputs a signal S1 of "L" level (voltage of zero) when the former is higher than the latter in level. Inversely, the amplifier 31 outputs a signal S1 of "H" level (voltage of V c ) when the former is lower than the latter in level. In this case, the threshold level V th is selected to a level lower than that of a pseudo-pulse occurring due to the mode hopping of the laser, as stated later. An output S1 of the amplifier 31 is input via the inverter 32, whose output is indicated by reference S2, to one input end of the AND gate 33. Another input end of the gate 33 receives the phase shifted clock pulse P4. An output of the AND gate 33 is input to the counter 27 as the discrimination signal S3. Next, the operation of the arrangement shown in Fig. 2 will be explained with reference to Fig. 3, and the waveform diagrams shown in Figs. 4a to 4g. First, assuming that a portion of light of the dominant mode is shifted by a wavelength of Δλ to the next mode (mode hopping). This passes through the optical fiber 3 and, as shown in Fig. 4a, forms a pseudo-pulse at the time position (time slot T3) delayed by a time of m·L·Δλ from the position of the sign of "1" (time slot T1) in the regenerative pulse pattern P2. The regenerative pulse pattern P2 is input via the equalizer 22 and timing extracting circuit 23 to the 1/N frequency dividing circuit 24, where the clock pulse P5 is divided in frequency into one eights (see Fig. 4b). Furthermore, the frequency divided clock pulse P3 is input to the phase shifting circuit 25, where it is shifted by the predetermined phase difference  from the above position of the sign of "1" (see Fig. 4c). In this case, a pulse width of the shifted pulse P4 may be selected broader than that of the original pulse signal P2, or narrower. Note, the pulse width is selected to be shorter than a time interval corresponding to the length subtracted by one bit from eight bits. Since Δλ is not predictable, the shifting phase difference  should be changed from T₂ through T₈ so that every expected positioned pseudo-pulse can be detected. The phase shifted pulse pattern P4 is input to one input end of the AND gate 33 in the discriminating circuit 26. On the other hand, the regenerative pulse pattern P2 is input via the equalizer 22 to the operational amplifier 31 in the discriminating circuit 26, where it is compared with the predetermined threshold level V th (see Figs. 4a and 4d). The signal S1 generated based on the comparison is input via the inverter 32 (see Fig. 4e) to another input of the AND gate 33. Therefore, the output of the AND gate 33, i.e., the output S3 of the discriminating circuit 26, presents "H" level at a time position corresponding to the position of occurrence of the pseudo-pulse, as shown in Fig. 4f. As a result, the counter 27 effects its counting operation. In this case, by measuring whether or not the counting operation is carried out while a preset number of pulses, e.g. 10¹¹ pulses, are generated by the pulse pattern generating circuit 11, it is possible to detect whether or not the mode hopping occurs in the semiconductor laser with an error rate of 10⁻¹¹. Namely, it becomes possible to discriminate whether or not the laser is functional. Also, it is possible to gain a value of mode hopping (Δλ) from the relation of /mL by making the width of the frequency divided clock pulse narrower than the original pulse width and gradually changing the phase difference  of the divided clock pulse to check up on the relation between the number of counts and the phase difference . In the embodiment, although the pulse pattern P1 is a set of 8 bits, such number of the bits can be selected depending upon the situation, i.e., m, L, Δλ. Although, in the described and illustrated embodiment, the arrangement is constituted to detect whether or not the mode hopping occurs in the semiconductor laser by carrying out a phase shift the clock signal P5 extracted from the regenerative pulse pattern P2 in the receiving side of the light transmission system, it is not restrictive. For example, it is also possible to cope with in the transmitting side of the system. An example in the case is shown in Figs. 5a to 5e, which illustrate a form of generation of the pulse pattern Pn employed in another embodiment of the present invention. A constitution of the arrangement (not shown) of the present example is substantially the same as that in Fig. 2, except that the timing extracting circuit 23, 1/N frequency dividing circuit 24 and phase shifting circuit 25 are unnecessary in the receiving equipment. However, the pulse pattern P1 is fed to the discriminating circuit 26 as shown in Fig. 2 with a broken line. That is, the original pulse pattern P1 is employed as the pulse pattern P4 to be input to the discriminating circuit 26, and an output gate is used which outputs a signal of "H" level when a combination of "1" and "0" is applied thereto. Also, the pulse pattern generating circuit 11 does not output an identical pulse pattern repeatedly with unit of a plurality of bits as in the embodiment of Figs. 2 and 4, but outputs a specific pulse pattern as shown in Figs. 5a to 5e, where the number of signs of "0" inserted into the adjacent signs of "1" is increased in sequence one by one from zero to a predetermined value (in the illustrated example, four). By utilizing the phase shifted pulse pattern, the following characteristics in operation are expected. When a portion of light of the dominant mode of the semiconductor laser is shifted by a wave length of Δλ (mode hopping), it forms a mode partition noise. When the noise passes through the optical fiber and is input to the receiving equipment, a pseudo-pulse is formed at the position delayed by the time of m·L·Δλ from the position of the sign of "1" in the regenerative pulse pattern. Where the pseudo-pulse occurs in the position of the sign of "1" in each pattern shown in Figs. 5a to 5e, it is treated as data of "1" in the receiving equipment, although it really indicated an error. Namely, it is brought to the state of "error free". In the present embodiment, when the phase of the pulse pattern to be applied to the semiconductor laser is changed in the manner as shown in Figs. 5a to 5e, the state of "error" and the state of "error free" are found in the receiving side. Accordingly, by monitoring for the presence of both the states, it becomes possible to exactly discriminate whether the occurrence of the error is due to the transmitting side (mainly , the mode hopping of the diode LD), or due to the receiving side (mainly, the noise of the diode APD). For example, assuming that, in the illustration of Fig. 5a to 5e, a time slot t1 is set as basis and a pseudo-pulse occurs in a time slot t3. A pulse pattern of Fig. 5a presents the state of "error free"; that of Fig. 5b "error free"; that of Fig. 5c "error"; that of Fig. 5d "error"; and that of Fig. 5e "error". As a result, it is found that the error occurs due to the transmitting side, mainly, the diode LD. If the error occur due to the receiving side, all of the patterns shown in Figs. 5a to 5e are brought to the state of "error". In other words, when the presence of both of the states "error" and "error free" is detected in the receiving side, it is possible to detect that the semiconductor laser of the transmitting side is "non-functional". Although the present invention has been disclosed and described by way of only two embodiments, it is apparent to those skilled in the art that other embodiments and modifications or the present invention are possible without departing from the essential features thereof.

Claims: An arrangement for detecting mode-hopping of a semiconductor laser, which arrangement comprises: a pattern generating circuit (11) for generating repeatedly a plural-bit pulse pattern (P1) to be applied to a semiconductor laser under test so as to modulate its optical output; and an optical receiving circuit (21) for employing such optical output, received by way of a transmission medium (3) producing chromatic dispersion, to produce a regenerated pulse pattern (P2); characterised by a discriminating circuit (26) connected for determining whether a pulse of an amplitude exceeding a predetermined threshold level (V th ) appears at a preselected position in said regenerated pulse pattern, thereby to indicate whether mode-hopping is occurring in the semiconductor laser. An arrangement as claimed in claim 1, wherein the said preselected position in the said regenerated pulse pattern (P2) is determined relative to a reference position in that pattern at which a "1"-bit appears, the said preselected position being spaced temporally from the said reference position by a preselected time interval which is determined in dependence upon the dispersion (m) and length (L) of the transmission medium (3) and upon wavelength fluctuation (Δλ) caused by mode-hopping of the semiconductor laser. An arrangement as claimed in claim 1 or 2, wherein the said plural-bit pulse pattern is a fixed-length sequence of bits produced at a preselected bit rate by the said pattern generating circuit (11), each sequence including a single "1"-bit, and wherein the said optical receiving circuit (21) comprises: a timing extracting circuit (23) for deriving from the optical output received by way of the said transmission medium (3) a clock signal (P5) of a frequency equal to the said preselected bit rate; a divider circuit (24) for dividing the clock signal frequency by a factor equal to the number of bits in each said sequence of bits so as to produce a frequency-divided clock signal; and a phase shift circuit (25) for producing gating pulses (P4) that are phase-shifted, relative to the clock pulses making up the said frequency-divided clock signal (P3), by a predetermined phase difference (); the said discriminating circuit (26) being operable to compare the regenerated pulse pattern amplitude with the said predetermined threshold level (V th ) during each gating pulse (P4). An arrangement as claimed in claim 3, wherein the said predetermined phase difference () corresponds to a time interval of a duration greater than, or equal to, one bit of the regenerated pulse pattern (P2). An arrangement as claimed in claim 1, wherein the number of "0"-bits between successive "1"-bits in the said plural-bit pulse pattern (P1) is initially zero but is incremented progressively by one, up to a predetermined maximum value, at intervals over the course of the pattern (P1). An arrangement as claimed in claim 5, wherein the pattern generating circuit (11) is connected to the said discriminating circuit (26) for applying the plural-bit pulse pattern (P1) directly to that circuit, the said discriminating circuit (26) being operable to detect any difference between that pattern (P1) and the regenerated pulse pattern (P2) at the said preselected position in the regenerated pulse pattern (P2). An arrangement as claimed in any preceding claim, further including a counter (27) connected to the said discriminating circuit (26) for counting the number of times mode-hopping is detected by the discriminating circuit (26). A method of detecting mode-hopping of a semiconductor laser, in which method a plural-bit pulse pattern (P1) is applied repeatedly to a semiconductor laser (LD) under test so as to modulate its optical output, and the optical output of the semiconductor laser, received by way of a transmission medium (3) producing chromatic dispersion, is employed to produce a regenerated pulse pattern (P2); characterised in that mode-hopping of the semiconductor laser is detected by determining whether a pulse of an amplitude exceeding a predetermined threshold level (V th ) appears at a preselected position in the said regenerated pulse pattern (P2).

Inventors: IMAI HAJIME, NISHIMOTO HIROSHI, IMAI, HAJIME, NISHIMOTO, HIROSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1988
Main classification: H04B10
IPC codes: H01S5, H04B10, H04L1