JP3464373B2 - Pulse light generator - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulsed light generator, and more particularly to a pulsed light generator suitable for use in an ultrahigh speed optical transmission system using optical pulses.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing an example of a conventional pulsed light generator. The operation of the conventional pulsed light generator will be described below with reference to FIG.

In FIG. 4, 41 is a reference signal oscillator and 4 is a reference signal oscillator.
2 is an optical pulse generator, and 43 is an optical amplifier. Reference numeral 44 is a low dispersion optical fiber, and 45 is an optical output port. The electrical signal generated by the reference signal oscillator 41 is input to the optical pulse generator 42 and used to generate an optical pulse having a constant repetition frequency. Usually, a sine wave generated from a low noise oscillator is used as this reference signal.

The optical pulse generator 42 generates an optical pulse having a frequency equal to the frequency of the input reference signal, an integral multiple thereof, or an integral fraction thereof. As the optical pulse generator 42, a mode-locked fiber ring laser, a mode-locked semiconductor laser, a gain modulation semiconductor laser, a collision pulse mode-locked laser, or the like can be used.

A mode-locked fiber ring laser will be described below with reference to FIGS.

FIG. 5 is a diagram showing a configuration example of a mode-locked fiber ring laser. FIG. 5A shows the basic configuration,
FIG. 5B shows a typical spectrum characteristic obtained by mode locking, and FIG. 5C shows a time response characteristic of the output waveform.

In FIG. 5A, reference numeral 51 is an oscillator, and 52 is an oscillator.
Is a power amplifier, and 53 is an optical modulator. 54 is an optical fiber, 55 is an optical amplifier, 56 is an optical isolator, and 57 is an output optical fiber coupler.

In FIG. 5A, the output of the oscillator 51 having a predetermined frequency is amplified by the power amplifier 52 and applied to the optical modulator 53. The optical modulator 53 modulates the loss or phase of light with the frequency of the oscillation output by the electro-optical effect. The optical amplifier 55 amplifies the modulated optical pulse. The optical isolator 56 defines the traveling direction of the optical pulse and blocks the reflected return light. The output optical fiber coupler 57 is used to take out the amplified optical pulse to the outside.

The modulator 53, the optical amplifier 55, the optical isolator 56, and the output optical fiber coupler 57 are coupled in a ring shape via the optical fiber 54, and thereby a ring resonator 58 is formed. As the optical amplifier 55, a rare earth-doped optical fiber amplifier mainly added with a rare earth such as erbium (Er) or neodymium (Nd),
A semiconductor laser amplifier is used.

FIG. 6 is a diagram showing a configuration example of a rare earth-doped optical fiber amplifier.

In FIG. 6, 61 is an Er-doped fiber, 62 is a wavelength multiplex wave device, and 63 is an excitation light source. An optical pulse is incident from one end (left end in the figure) of the rare earth-doped optical fiber, and an amplified optical pulse is emitted from the other end (right end in the figure). The excitation light from the excitation light source 63 is incident on the rare earth-doped optical fiber through the wavelength multiplex wave multiplexer 62 connected to the emission end, the incidence end, or both ends of the optical pulse of the rare earth-doped optical fiber. In the rare earth-doped optical fiber, the optical pulse is amplified by this excitation light.

FIG. 6 (a) shows a backward pumping configuration in which the wavelength multiplexing polymer wave 62 is connected to the emitting end of the rare earth-doped optical fiber, and FIG. 6 (b) shows the wavelength multiplexing polymer wave 62 of the rare earth-doped optical fiber. A forward excitation configuration connected to the entrance end is shown in FIG.
(C) shows a bidirectional pumping configuration in which the wavelength multiplexing polymer wave 62 is connected between the entrance end and the exit end of the rare earth-doped optical fiber.

Next, referring to FIG. 7, a semiconductor laser amplifier which can constitute the optical amplifier 55 will be described.

FIG. 7 is a diagram showing a configuration example of a semiconductor laser amplifier.

In FIG. 7, reference numeral 71 is a drive power source (current source).
, 72 are semiconductor laser amplifiers. The semiconductor laser amplifier 72 is configured to amplify the input light pulse according to the current supplied from the drive power supply 71 and output the amplified light pulse.

Now, returning to FIG. 5, the operation principle of the mode-locked fiber ring laser will be described.

When the optical path length of the ring resonator 58 is L, the physical length of each component (53, 55, 56, 57) of the ring resonator 58 is h, and the refractive index of each component is n, the optical path length L
Is the sum of the values obtained by multiplying the respective physical lengths h _i by the respective refractive indices n _i (that is, the respective optical path lengths), and is calculated by the following equation (1).

[0018]

## EQU1 ## L = Σh _i n _i (1) By the way, in the ring resonator 58 of FIG. 5, fr = c / L
There are a number of longitudinal modes with a frequency spacing given by (where c is the speed of light).

Here, the optical modulator 5 in the ring resonator 58
When the optical modulation with the repetition frequency fm = N × fr (where N is an integer of 1 or more) is performed by 3, the phases of all the longitudinal modes of the frequency interval N × fr are aligned as shown in FIG. 5B. The mode-locked oscillation state is set. Also at this time, FIG.
As shown in (c), the repetition frequency is 1 / (N × f
The optical pulse train of r) is obtained. The pulse width of each optical pulse corresponds to the reciprocal of the oscillation spectrum width δν determined by the envelope of many longitudinal mode spectra, and the center of the envelope of this spectrum is the central wavelength (center frequency νo) (FIG. (See (b)).

Next, a mode-locked semiconductor laser and a gain-modulated semiconductor laser which can constitute the optical pulse generator 42 will be described with reference to FIGS. 8 and 9.

FIG. 8 is a diagram showing a configuration example of a mode-locked semiconductor laser.

In FIG. 8, 81 is a semiconductor laser and 82
Reference numerals 84 and 84 are lenses, and 83 is a light intensity modulator. Reference numeral 85 is a reflecting mirror, 86 is an oscillator, and 87 is an output light extracting half mirror. The light intensity modulator 83 is located at the center of the optical path between the reflecting mirror 85 and the output light extracting half mirror 87 in which the optical pulse reciprocates. The optical pulse is driven at a frequency twice as high as the frequency that reciprocates in this optical path, or a frequency equal to an integral multiple thereof, so that the mode-locked operation is realized by the same principle as that of the mode-locked fiber ring laser. Generate.

The semiconductor laser 81 and the light intensity modulator 8
3 and the reflecting mirror 85 are monolithically integrated on the same semiconductor substrate, lenses 82 and 84 in FIG.
Is unnecessary.

Further, as shown in FIG.
By arranging the configuration of (a) symmetrically and disposing the saturable absorbing medium 88 in the center, a collision pulse mode-locked laser capable of generating a short optical pulse can be constructed.

FIG. 9 is a diagram showing a configuration example of a gain modulation semiconductor laser.

In FIG. 9, 91 is an oscillator, 92 is an amplifier, 93 is a DC power supply, 94 is a direct bi-bias circuit, and 95 is.
Is a semiconductor laser, 96 is a normal dispersion fiber, and 97 is an output optical terminal.

The output of the oscillator 91 is amplified by the amplifier 92. The amplified output is superposed on the DC voltage from the DC power supply 93 by the bias circuit 94 and input to the semiconductor laser 95. Oscillation frequency of oscillator 91 is 1 GHz
In the case of above a certain level, the semiconductor laser 95 generates a short light pulse of about 30 ps by relaxation oscillation. The optical pulse generated by the semiconductor laser 95 is output from the output optical terminal 97.

Here, the optical pulse generated by the semiconductor laser 95 has frequency chirping in which the wavelength changes in the long wavelength direction with the passage of time of the pulse. In order to remove this frequency chirping, the generated optical pulse may be output through the normal dispersion fiber 96. Pulse width is about 10p due to normal dispersion fiber 96
It is compressed to s and output from the output optical terminal 97.

Returning to FIG. 4, the optical pulse generated from the optical pulse generator 42 has the following two characteristics.

1) The pulse width is 10 ps or less.

2) The time bandwidth product of the pulse is within twice the value for the transform limit (TL) light pulse.

In particular, a mode-locked fiber ring laser,
A mode-locked semiconductor laser or the like can generate almost perfect TL light pulses.

After the optical pulse having these characteristics is amplified by the optical amplifier 43, it is incident on the low dispersion optical fiber 44.
The dispersion value of the low-dispersion optical fiber 44 is close to the wavelength of the optical pulse generated by the optical pulse generator 42 (about 20 n).
It has been chosen to have zero variance within m).

When an optical pulse obtained by amplifying the optical pulse generated by the optical pulse generator 42 is incident on the low dispersion optical fiber 44 satisfying the above condition, the optical spectrum spreads in the low dispersion optical fiber 44 due to the optical nonlinear effect. Phenomenon (Super Continuum; SC) is seen. SC
Then, the spread width of the spectrum reaches 200 nm or more. The characteristic of the supercontinuum phenomenon is that the coherence of light is at least 10n in the broad spectrum.
The point is that m or more is maintained.

Therefore, when the spectrum spread by this SC is cut out by an optical bandpass filter having a wider bandwidth than the spectral width of the incident light pulse, an optical pulse having a pulse width narrower than the pulse width of the incident light pulse is generated. be able to. It has been confirmed that this phenomenon can be used to generate an extremely short pulse having a pulse width of several ps to 170 fs.

Detailed experimental results of the generation of ultrashort pulses are as follows.
“T. Morioka, S. Kawan _i sh _i , K.
Mori, and M.M. Saruwatari "Tra
nsform-limited, femtosecond
d WDM pulse generation by
spectral filtering of gi
gahertz supercontinuum ”, E
electron. Lett "vol., 30, pp. 1
166-1168, 1994. "It is described in.

[0037]

However, the above-mentioned prior art has the following problems. That is, in order to reduce the pulse width of the output optical pulse to 100 fs or less, it is necessary to widen the spectrum band to be cut out to 10 nm or more, but when the spectrum band exceeds 10 nm, the long wavelength side of the spectrum of the generated SC light is generated. Since the power fluctuation and the phase difference occur on the short wavelength side, it is difficult to reduce the pulse width in the time response of the optical pulse even if the band to be cut out is further expanded.

Therefore, the present invention has been made in view of the above-mentioned problems of the prior art, and the generated SC light is a bright line having a frequency interval corresponding to the repetition frequency of the incident light pulse generated therein. By extracting each spectral component separately, adjusting the optical intensity and optical phase independently for each component, and then combining each component to minimize the pulse width of the output optical pulse, the time width is extremely small. It is an object of the present invention to provide a pulsed light generator capable of generating a narrow light pulse.

[0039]

In order to achieve the above object, in the device of the present invention as defined in claim 1, an optical pulse generating means for generating an optical pulse repeated at a predetermined frequency, and a wavelength of the optical pulse are provided. Pulsed light generation having an optical means having zero dispersion in the vicinity of, and generating supercontinuum light having a spectral component of a frequency interval corresponding to the repetition frequency by incident light based on the optical pulse. In the device, the incident based on the light pulse, which constitutes the supercontinuum light
A spectrum with a frequency interval corresponding to the repetition frequency of light.
Separating means for separating and selectively transmits the spectral components in each torr component, by adjusting the independently magnitude and phase for each spectral component and the separation, the intensity of each spectral component constant In addition, it is characterized by comprising adjusting means for minimizing the phase difference between the spectral components and optical multiplexing means for multiplexing the spectral components obtained by the adjusting means.

Here, in the apparatus of the present invention as set forth in claim 2, each of the optical means, the separating means, the adjusting means, and the optical multiplexing means can have a polarization maintaining characteristic. .

[0041]

According to the apparatus of the present invention having the above-mentioned structure, the generation condition of the short optical pulse can be satisfied in a wider spectrum range of the supercontinuum light generated by the optical means.

Further, in the apparatus of the present invention having the above-mentioned structure, the polarization state in the process after the optical means can be kept constant.

[0043]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a diagram showing a first embodiment of an optical pulse generator to which the present invention is applied.

In FIG. 1, 101 is a reference signal oscillator,
102 is an optical pulse generator, 103 is an optical amplifier, 104
Is a low dispersion optical fiber, 105 is a periodic optical filter, 106
Is an optical output port.

The electrical signal generated by the reference signal oscillator 101 is input to the optical pulse generator 102 and used to generate an optical pulse having a constant repetition frequency.
Usually, a sine wave generated from a low noise oscillator is used as this reference signal.

The optical pulse generator 102 generates an optical pulse having a frequency equal to the frequency of the input reference signal, an integral multiple thereof, or an integral fraction thereof. As the optical pulse generator 102, as in the optical pulse generator 42 shown in FIG. 4, a mode-locked fiber ring laser, a mode-locked semiconductor laser, a gain modulation semiconductor laser,
A collision pulse mode-locked laser or the like can be used.

Therefore, the optical pulse generated from the optical pulse generator 102 also has the above-mentioned two characteristics (the pulse width is 10 ps or less, and the time bandwidth product of the pulse is a transform limit (TL) optical pulse). It should be within 2 times the value in.

After the optical pulse having these characteristics is amplified by the optical amplifier 103, it is incident on the low dispersion optical fiber 104. The low dispersion optical fiber 104 is selected so that its dispersion value has zero dispersion in the vicinity (within about 20 nm) of the wavelength of the optical pulse generated from the optical pulse generator 102, and as described in the prior art. In the low dispersion optical fiber 104, a supercontinuum phenomenon occurs,
C light is generated.

In order to generate a short optical pulse having a width narrower than that of the method using the prior art, in the present embodiment, the generation condition of the short optical pulse is satisfied for the SC light in a wider spectrum range. The periodic optical filter 105 controls the internal structure of the SC optical spectrum. The periodic optical filter 105 can be composed of an arrayed waveguide grating filter.

FIG. 2 is a diagram showing the structure of the arrayed waveguide grating filter.

In FIG. 2, 201 denotes an optical input port and 2
Reference numeral 02 is an array waveguide grating for demultiplexing. 203-1 ~ 2
03-k (k is a natural number) is a variable attenuator, 204
-1 to 204-k are variable phase adjusters, respectively. Further, 205 is an array waveguide grating for multiplexing, and 206 is an optical output port.

The operation of the arrayed waveguide grating filter shown in FIG. 2 will be described below. The light incident from the optical input port 201 is divided into different wavelength components of the incident light by the demultiplexing arrayed waveguide grating 202 and output from the k output terminals. That is, the bright line spectrum components forming the SC light generated in the low-dispersion optical fiber 104 are cut out for each component.

FIG. 3 shows one of the demultiplexing array waveguide gratings 202.
It is a characteristic view which shows the frequency (wavelength) transmission characteristic of the output terminal of the kth to the kth.

Assuming that fo is the oscillation frequency of the reference signal oscillator 101, the transmission center wavelength (frequency) of each output is output from each of the output terminals 1 to k of the demultiplexing arrayed waveguide grating 202, as shown in FIG. , The transmitted light frequency of the shortest wavelength port is f
A bright line spectrum component shifted by fo is transmitted as c. In this way, the input SC light is selectively transmitted with a good extinction ratio for each bright line spectrum component of the frequency interval corresponding to the repetition frequency of the incident light pulse generated inside, and separated into each bright line component. To be done. Furthermore, this k
The bright line spectral components of the book are independently variable attenuators 203-1 to 203-k and variable phase adjuster 20.
After the intensity and the phase are adjusted by 4-1 to 204-k, they are combined again by the combining array waveguide grating 206.

Here, the variable attenuators 203-1 to 203-
By adjusting the intensity and the phase by the k and variable phase adjusters 204-1 to 204-k, the optimum condition is set for the generation condition of the ultrashort optical pulse.

At this time, when the amplitudes of the k bright line spectrum components are equal and there is no phase difference, the optical pulse S (t) recombined by the multiplexing array waveguide grating 206 is used. ) Is represented by the following equation (2).

[0058]

[Equation 2]

The above equation is equivalent to the expansion of the periodic impulse waveform into the Fourier series. That is, it means that the pulse width of the re-synthesized light pulse S (t) is extremely narrow.

As an example, assuming that the envelope of the synthesized bright line spectrum sequence is Gaussian and the half width is Δλ, the pulse width in the time response of the recombined optical pulse is calculated.

Here, assuming that Δλ = 100 nm and the generated optical pulse is a transform-limited optical pulse, the time bandwidth product of the optical pulse becomes a constant value (0.44). Since the wavelength band 100 nm of the half width is 12 THz when converted into the frequency band, the time width to be obtained is 3.6 × 10 ⁻¹⁴ [s], that is, 36 [f
s]. This time pulse width can be set to a value of about ⅕ of the value achieved by the prior art.

As described above, according to this embodiment, the SC light in a wider spectrum range is adjusted so that the generation condition of the short optical pulse is satisfied and the pulse width of the output optical pulse is minimized. It is possible to generate an optical pulse having an extremely narrow time width and generate an optical pulse having a pulse width narrower than that of the conventional one. Therefore, when the device of the present invention is used as a wavelength tunable ultrashort optical pulse light source in ultrahigh-speed optical transmission or optical signal processing, remarkable effects can be obtained.

The order in which the separated bright line spectrum components pass through the variable attenuators 203-1 to 203-k and the variable phase adjusters 204-1 to 204-k is configured so as to be opposite to that in FIG. You may.

Further, in the present embodiment, since the amplitude and phase of each separated bright line spectrum can be controlled independently, it is possible to freely generate optical pulses other than the Gaussian optical pulse mentioned in the example. You can

(Other Embodiments) Of the components shown in FIGS. 1 and 2, the low-dispersion optical fiber 104, the demultiplexing array waveguide grating 202, and the variable attenuators 203-1 to 203-3 are included.
The -k, the variable phase adjusters 204-1 to 204-k, and the multiplexing array waveguide grating 205 can each have a polarization maintaining configuration.

According to the above configuration, the low dispersion optical fiber 1
Since all the components in the process from the generation of supercontinuum light by 04 to the finally combined output by the periodic optical filter 105 are polarization-maintaining structures, the polarization state in this process is It can be held constant. Therefore, it becomes possible to always stably generate the short light pulse.

[0067]

As described above, according to the first aspect of the invention, the spectrum of the frequency interval corresponding to the repetition frequency generated by the incidence of the light pulse for generating the light pulse repeating at the predetermined frequency is incident. Based on optical pulse with spectral component of supercontinuum light with component
A spectrum with a frequency interval corresponding to the repetition frequency of the incident light.
Tol components are selectively transmitted and separated, and the intensity and phase are adjusted independently for each separated spectral component to satisfy the condition of ultrashort pulse generation, and each spectral component obtained by adjustment By multiplexing the above, it is possible to generate an optical pulse having an extremely short time width, and it is possible to provide an optimum device as a wavelength-tunable ultra-short optical pulse light source in ultra-high-speed optical transmission and optical signal processing.

According to the second aspect of the invention, since the polarization maintaining structure is provided after the optical means for generating the supercontinuum light, the polarization state in the process after the optical means is maintained constant. Therefore, there is an effect that a light pulse having an extremely short time width can always be stably generated.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of an optical pulse generator to which the present invention is applied.

FIG. 2 is a diagram showing a configuration of an arrayed-waveguide grating filter that is a component of the device according to the first embodiment.

FIG. 3 is a diagram showing frequency (wavelength) transmission characteristics of each output terminal in the arrayed waveguide grating filter.

FIG. 4 is a configuration diagram showing an example of a conventional pulsed light generator.

FIG. 5 is a diagram showing a configuration example of a mode-locked fiber ring laser.

FIG. 6 is a diagram showing a configuration example of a rare earth-doped optical fiber amplifier.

FIG. 7 is a diagram showing a configuration example of a semiconductor laser amplifier.

FIG. 8 is a diagram showing each configuration example of a mode-locked semiconductor laser.

FIG. 9 is a diagram showing a configuration example of a gain modulation semiconductor laser.

[Explanation of symbols]

101 Reference signal oscillator 102 optical pulse generator 103 Optical amplifier 104 Low dispersion optical fiber 105 periodic optical filter 106,206 Optical output port 201 Optical input port 202 Demultiplexing array waveguide grating 203-1 to 203-k variable attenuator 204-1 to 204-k variable phase adjuster 205 Array waveguide grating for multiplexing

Continuation of the front page (56) References JP-A-9-236834 (JP, A) JP-A-7-79212 (JP, A) T.M. Morioka et al, 1 Tbit / s (100 Gbit / s * 10 c channel) OTDM / WDM transmission using a single superindium WDM source, Eltronics Letters, May 5, 1996. 32, no. 10, pp. 906-907 (58) Fields surveyed (Int.Cl. ⁷ , DB name) G02F 1/29-7/00 JISST file (JOIS)

Claims (2)

(57) [Claims] 1. An optical pulse generating means for generating an optical pulse repeating at a predetermined frequency, and an optical means having zero dispersion in the vicinity of the wavelength of the optical pulse, wherein the repeating frequency is generated by incident light based on the optical pulse. in the pulse light generating device and an optical means for generating a supercontinuum light having spectral components corresponding to the frequency interval, constituting the supercontinuum light, the light path
Corresponding to the repetition frequency of the incident light based on
A separating means for selectively transmitting and separating the spectral components for each spectral component of a frequency interval , and the intensity of each spectral component by independently adjusting the intensity and phase for each of the separated spectral components. Pulse light generation characterized in that it comprises: adjusting means for minimizing the phase difference between the spectral components and optical combining means for multiplexing the spectral components obtained by the adjusting means. apparatus. 2. The pulsed light generator according to claim 1, wherein the optical unit, the separating unit, the adjusting unit, and the optical combining unit each have a polarization maintaining characteristic.