JPH0774730A - Multi-frequency light source - Google Patents

Abstract

PURPOSE:To attain miniaturization by using a semiconductor laser able to select an optical frequency at a high speed for an optical oscillator. CONSTITUTION:An optical oscillator 10 of a semiconductor laser able to switch an optical frequency at a high speed such as a DFB laser provides an output of lights whose frequency are f1, f2,...,fN sequentially at a switching time T and a period NT based on a control signal outputted from an optical frequency control means 11. The light is branched into N by an optical branch means 12 and an optical delay means 13 gives delay times of 0, T, 2T,..., (N-1) T (or T, 2T, 3T,...,NT) to each branched light. An optical synthesizer 14 synthesizes the lights. Thus, one optical oscillator outputs simultaneously lights of two optical frequencies. Thus, even when the oscillator is mounted on a transmitter in, e.g. an optical path changeover system, since number of optical oscillators is remarkably reduced, the cost is reduced by miniaturization and the reliability is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-frequency light source which simultaneously outputs light of a plurality of optical frequencies.

[0002]

2. Description of the Related Art A conventional multi-frequency light source for simultaneously outputting light of a plurality of optical frequencies has an optical oscillator corresponding to each optical frequency.

Here, an optical path switching system that requires a multi-frequency light source will be described with reference to FIG. This system uses optical frequencies as routing information, and each optical frequency is assigned to each multiplexed optical transmission line.

In FIG. 9, a transmitter 100 and a receiver 200 are connected via optical transmission lines 301 and 302, and the route between the devices is duplicated. Normally, one is used as the active system and the other is used as the standby system. It should be noted that one of the optical transmission lines 302 is connected to the repeater 310, and the delay time of each optical transmission line is different. The delay adjustment circuit 201 of the reception device 200 absorbs the delay time difference of each optical transmission line and gives each reception signal to the switching circuit 202. As a result, the switching circuit 202 can switch the path to the standby system without interruption in response to a failure in the active system.

At this time, by assigning an optical frequency corresponding to each optical transmission line, the path of the received signal can be identified in the receiving apparatus 200. For this reason, the transmitter 100 is provided with optical oscillators having different oscillation optical frequencies, and is configured to correspond to each optical transmission line.

FIG. 10 shows an example of the configuration of the transmitting apparatus 100. In the figure, a transmitter 100 includes an optical oscillator 101, which outputs light of optical frequencies f ₁ and f ₂ corresponding to each optical transmission line.
102 and optical combining means 103 for combining light of each optical frequency
, A signal source 111 for outputting a transmission signal, and the optical multiplexing means 1
Optical modulator 112 which modulates the output light of the optical signal 03 of FIG.
And an optical frequency separating means 113 for separating and outputting an optical signal having an optical frequency corresponding to each optical transmission line.

[0007]

In the transmitter 100 in the optical path switching system described above, the number of optical oscillators corresponding to the number of multiplexed optical transmission lines is required as a multi-frequency light source. Further, in the case of duplicating the optical oscillator for one path in order to ensure reliability, the optical oscillator of double the number is required.

As described above, the conventional multi-frequency light source needs to individually include an optical oscillator corresponding to each optical frequency,
This has been a major factor in increasing the circuit scale and cost of the transmitter and the like in the optical path switching system.

An object of the present invention is to provide a multi-frequency light source capable of simultaneously outputting light of a plurality of optical frequencies by using one optical oscillator.

[0010]

A multi-frequency light source of the present invention includes an optical oscillator that outputs light having an optical frequency corresponding to a control signal and a control signal that sets a plurality N of optical frequencies within a predetermined switching time T. Optical frequency control means for sequentially switching and giving to the optical oscillator, optical branching means for branching the output light of the optical oscillator to the switching number N of the optical frequency, and predetermined switching time T for each output light of the optical branching means. The optical delay means for sequentially giving a delay time corresponding to the optical delay means and the optical multiplexing means for multiplexing and outputting the respective output lights of the optical delay means.

[0011]

1 shows the basic construction of the multi-frequency light source of the present invention. In the figure, an optical oscillator 10 is an optical frequency control means 1
1, the optical frequencies f ₁ , f ₂ ,
Lights of f ₃ , ..., F _N are sequentially output at a switching time T and a cycle NT. The light splitting means 12 splits this light into N branches. The optical delay means 13 applies 0, T, 2T, to each of the N branched lights.
..., (N-1) T (or T, 2T, 3T, ..., NT)
Give a delay time. The light combining means 14 combines these lights.

The operating principle of the multi-frequency light source of the present invention when N = 2 will be described with reference to FIG. The horizontal axis is time. Is a control signal for setting the optical frequencies f ₁ and f ₂ in the optical oscillator 10, and the corresponding levels I ₁ and I ₂ are repeated at the switching time T and the cycle 2T. Is
The change in the optical frequency of the light output from the optical delay unit 13 with the delay time of 0 is shown. Indicates a change in the optical frequency of the light having the delay time T output from the optical delay unit 13. Indicates a change in the optical frequency of the output light of the optical multiplexer 14. In this way, the switching time T for sweeping the optical frequency of the optical oscillator 10 and the delay time T in the optical delay means 13 are associated with each other, and the optical multiplexing means 14 is used.
By combining the output lights of the optical delaying means 13 with each other, continuous lights of optical frequencies f ₁ and f ₂ can be obtained.

Next, a general example is shown in FIG. Is a control signal for setting the optical frequency f ₁ ~f _N to the optical oscillator 10, the corresponding level I ₁ ~I _N switching time T, the period N
Repeat with T. Indicates a change in the optical frequency of the light output from the optical delay unit 13 with a delay time of zero. Indicates a change in the optical frequency of the light having the delay time T output from the optical delay unit 13. Is the delay time 2T output from the optical delay unit 13.
Shows the change in the optical frequency of the light. Indicates a change in the optical frequency of the light having the delay time (N−1) T output from the optical delay unit 13. Indicates a change in the optical frequency of the output light of the optical multiplexer 14. Thus, the optical frequency f ₁ is output from the optical multiplexer 14.
Continuous light of up to f _N can be obtained.

[0014]

FIG. 4 shows the structure of a first embodiment of a multi-frequency light source according to the present invention. In the figure, the oscillation light frequency of the semiconductor laser 20 changes according to two current values supplied from the variable current source 21. The output light of the semiconductor laser 20 is branched into two by the optical fiber coupler 22, one branched output is kept as it is, and the other branched output is multiplexed by the optical fiber coupler 24 via the optical fiber delay line 23 of length L. The semiconductor laser 20, variable current source 21, optical fiber coupler 2
2, the optical fiber delay line 23, the optical fiber coupler 24,
They correspond to the optical oscillator 10, the optical frequency control means 11, the optical branching means 12, the optical delay means 13, and the optical multiplexing means 14, respectively.

The variable current source 21 has two current values I ₁ and I ₁ .
₂ is alternately output repeatedly at a switching time T and a cycle 2T (FIG. 2). At this time, the oscillation light frequency of the semiconductor laser 20 is such that frequencies f ₁ and f ₂ are alternately repeated with a cycle of 2T (FIG. 2). Here, in order to perform stable optical frequency switching, the transmission rate Rb, the time Tr required for switching the optical frequency of the semiconductor laser 20, and the time at which an optical frequency drift due to a change in the amount of heat generated due to a change in the injection current into the semiconductor laser 20 appears. If Tm, it is necessary to satisfy the relationship of Rb.Tr << 1 and Tr <T <Tm.

When a DFB laser is used as the semiconductor laser 20, the time Tr required for switching the optical frequency is several hundred ps.
Very fast with (eg S.Kuwano et al., I
EEEPhotonics Technology Letters, vol.5, No.3, pp.354-
356, 1993). Therefore, when the transmission rate Rb is up to several Gb / s, Rb · Tr << 1 is established, and it can be considered that the optical frequency switching is instantaneously performed. Further, the time Tm in which the optical frequency drift appears due to the change in the amount of heat generated due to the change in the injection current to the semiconductor laser 20 is about several hundred ns after the current value is switched. Therefore, by changing the optical frequency switching time T sufficiently smaller than several hundreds of ns to switch the current value before the occurrence of the optical frequency drift, and by keeping the calorific value per cycle constant, the calorific value change Optical frequency drift can be suppressed.

The length L of the optical fiber delay line 23 is set so that L = cT, where c is the propagation velocity (group velocity) of light in the optical fiber. As a result, the optical fiber delay line 23 can provide a delay time corresponding to the switching time T (= L / c) (FIG. 2). Therefore, if the light that has passed through the optical fiber delay line 23 and the light that does not pass are multiplexed by the optical fiber coupler 24, continuous light of the optical frequencies f ₁ and f ₂ can be obtained (FIG. 2). Here, when the switching time and the delay time T are set to 20 ns, the output waveform of the multi-frequency light source (optical fiber coupler 24) of this embodiment is shown in FIG. The horizontal axis represents time and the vertical axis represents light intensity. Optical frequency f ₁ ,
It can be seen that the light of f ₂ is simultaneously and continuously output.

As described above, in the multi-frequency light source of this embodiment, by using the semiconductor laser capable of high-speed optical frequency switching such as the DFB laser as the optical oscillator,
Light of two optical frequencies can be simultaneously output by the optical oscillator of the table. Moreover, all of the optical fiber couplers 22 and 24 and the optical fiber delay line 23, which constitute the multi-frequency light source of this embodiment, are passive optical components and are characterized by requiring no control other than temperature stabilization.

An optical modulator 112 for superposing the transmission signal given from the signal source 111 on the output light of the optical fiber coupler 24 and an optical frequency separating means 113 for separating the optical signals of the optical frequencies f ₁ and f ₂ are provided. As a result, it is possible to realize the same device as the transmitting device 100 in the optical path switching system shown in FIG.

The optical modulator 112 is, for example, LiNbO.
₃ A waveguide type intensity modulator on the substrate is used. As the optical frequency separating means 113, for example, an arrayed waveguide type optical filter shown in FIG. 6 (Takahashi et al., 1991 Autumn Meeting of the Institute of Electronics, Information and Communication Engineers, C-200) is used. The arrayed waveguide type optical filter guides the light input from the input optical fiber 31 to the arrayed waveguide 33 via the slab waveguide 32, and combines the light passing through the arrayed waveguide 33 with the slab waveguide 34. It is a configuration for sending to the output optical fiber 35. In the arrayed waveguide type optical filter, the position of the output optical fiber 35 is selected according to the position of the input optical fiber 31 and the wavelength of the input light. In this embodiment, since one input optical fiber 31 is used, the output lights of optical frequencies f ₁ and f ₂ can be separated into fixed positions.

FIG. 7 shows the reception error rate characteristics for each output light when the transmission rate Rb is 156 Mb / s and the optical frequency difference ΔF between the output light 1 and the output light 2 is 10 GHz. The horizontal axis represents the received optical power, and the vertical axis represents the bit error rate. As shown in the figure, it can be seen that the same characteristics as the reception error rate characteristics of the conventional device are obtained.

FIG. 8 shows the structure of a second embodiment of the multi-frequency light source of the present invention. This embodiment is generally an extension of the configuration example of the first embodiment where N = 2. Therefore, in the configuration of the first embodiment, the optical fiber coupler 2
Instead of 2, 24, optical star couplers 42 and 44 that perform 1-to-N branching and multiplexing are used, and lengths L, 2L, 3L, ..., (N
-1) L (N-1) optical fiber delay lines 43 ₁ to 43
_{It is the same except that N-1} is used.

The oscillation light frequency of the semiconductor laser 20 changes according to the current value supplied from the variable current source 21. The output light of the semiconductor laser 20 is N-branched by the optical star coupler 42, one of the branched outputs is left as it is, and the other branched output is combined by the optical star coupler 44 via each of the optical fiber delay lines 43 ₁ to 43 _N-1. Be waved.

The variable current source 21 has two current values I _{1 to} I.
_N is repeatedly output in order at the switching time T and the cycle NT (FIG. 3). Here, the switching time T is set so as to satisfy Tr <T <Tm as in the first embodiment. At this time, the semiconductor laser 2
The oscillating light frequency of 0 is frequencies f ₁ , f ₂ , f ₃ , ..., F
_N is sequentially repeated in a cycle of 2T (Fig. 3).

(N-1) optical fiber delay lines 43 ₁ to 4 3
In 3 _N-1 , delay times of T, 2T, 3T, ..., (N-1) T are given to the lights branched by the optical star coupler 42 (FIGS. 3 to). Therefore, the optical star coupler 4
If the light that has passed through the optical fiber delay lines 43 ₁ to 43 _N-1 and the light that does not pass at 4 are multiplexed, continuous light with optical frequencies f _{1 to} f _N can be obtained (FIG. 3). As described above, in the multi-frequency light source of the present embodiment, by using the semiconductor laser capable of high-speed optical frequency switching such as the DFB laser as the optical oscillator, the optical frequency f ₁ to
The light of f _N can be output at the same time.

Further, in the transmitting device 100 of the optical path switching system shown in FIG. 10, when the multi-frequency light source of this embodiment is used, the optical frequency separating means 113 has the optical frequency f ₁
The one that separates the optical signals of ~ f _N is used. Note that FIG.
The arrayed waveguide type optical filter shown in can be used.

[0027]

As described above, the multi-frequency light source of the present invention can simultaneously output light of a plurality of optical frequencies by using one optical oscillator. Therefore, for example, even when mounted on a transmitter in an optical path switching system,
Since the number of optical oscillators can be significantly reduced, cost reduction and reliability improvement due to miniaturization can be achieved.

[Brief description of drawings]

FIG. 1 is a block diagram showing a basic configuration of a multi-frequency light source of the present invention.

FIG. 2 is a diagram for explaining the operation principle of the multi-frequency light source of the present invention when N = 2.

FIG. 3 is a diagram illustrating a general operation principle of the multi-frequency light source of the present invention.

FIG. 4 shows a configuration of a first embodiment of a multi-frequency light source of the present invention.

FIG. 5 is a diagram showing an output waveform of the multi-frequency light source of this embodiment.

FIG. 6 is a diagram showing a configuration example of an arrayed waveguide type optical filter used as an optical frequency separation means 113.

FIG. 7 is a diagram showing a reception error rate characteristic for each output light of the present embodiment.

FIG. 8 shows a configuration of a second embodiment of the multi-frequency light source of the present invention.

FIG. 9 is a block diagram showing a configuration example of an optical path switching system that requires a multi-frequency light source.

FIG. 10 is a block diagram showing a configuration example of a transmission device 100.

[Explanation of symbols]

 10 optical oscillator 11 optical frequency control means 12 optical branching means 13 optical delay means 14 optical combining means 20 semiconductor laser (LD) 21 variable current source 22, 24 optical fiber coupler 23, 43 optical fiber delay line 42, 44 optical star coupler

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Kiyoshi Nosu 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. An optical oscillator for outputting light having an optical frequency according to a control signal, and an optical frequency control means for sequentially switching control signals for setting a plurality of optical frequencies at a predetermined switching time and giving the optical oscillator. Optical branching means for branching the output light of the optical oscillator into the number of switching optical frequencies, and optical delay means for sequentially giving a delay time corresponding to the predetermined switching time to each output light of the optical branching means. A multi-frequency light source, comprising: and an optical multiplexing unit that multiplexes and outputs each output light of the optical delay unit.