JP3331554B2 - Laser pulse oscillator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser pulse oscillator for stably generating a high repetition optical pulse required for constructing an ultra-high-speed optical communication system.

[0002]

2. Description of the Related Art In recent years, researches for generating a high repetition optical pulse by an optical fiber laser utilizing a mode locking technique have been actively conducted.

FIG. 1 shows an example of a conventional laser pulse oscillator. In FIG. 1, reference numeral 1 denotes an optical fiber doped with a rare earth element (hereinafter, referred to as a rare earth doped optical fiber), 2 denotes an excitation light source for exciting the rare earth doped optical fiber 1, and 3 denotes a rare earth doped optical fiber 1 An optical coupler for coupling the laser output to the optical coupler;
5 is an optical isolator that limits the traveling direction of light to one direction,
6 is an optical modulator, 7 is an optical filter, 8 is a synthesizer, 9
Is an electric signal amplifier.

In the laser pulse generator shown in FIG. 1, an optical pulse is generated as follows. When the rare-earth-doped optical fiber 1 is excited by the excitation light source 2 through the optical coupler 3, continuous light oscillation occurs in the forward direction of the optical isolator 5 within the transmission band of the optical filter 7. Next, the electric signal output from the synthesizer 8 is applied to the optical modulator 6 through the amplifier 9.

In general, when the length of the resonator is L, the refractive index of the optical fiber is n, and the speed of light is c, modulation at a frequency f ₀ = c / nL determined by the length of the resonator results in mode-locking with a fundamental wave. Is realized, and a stable optical pulse train can be generated.

However, in this case, the modulation frequency is q times the fundamental frequency determined by the resonator length of the laser, and qf ₀ = qc / n
When set to L (q is an integer), forced mode locking of harmonics oscillating at a frequency q times the fundamental wave can be realized. That is,
Q optical pulses are created at equal intervals in the laser cavity,
An optical pulse train having a repetition corresponding to the higher-order modulation frequency is generated. This optical pulse train is output through the optical splitter 4.

For example, when the cavity length of the laser is 20 m, the fundamental frequency determined by the cavity length is 10 MHz, but when the modulation frequency is set to q = 1000 and 10 GHz, an optical pulse train having a repetition of 10 GHz is set. Can occur.

[0008]

In such a conventional harmonic mode-locked laser pulse oscillator, modulation is performed at q times the fundamental frequency f ₀ (longitudinal mode) determined by the length of the resonator, so that the mode interval is qf. The phases between the longitudinal modes separated by ₀ are fixed.

However, there are q combinations of longitudinal modes whose mode intervals are separated by qf ₀ , and form super modes independent of each other. FIG. 2 shows an example of the super mode when q = 3.

In FIG. 2, many longitudinal modes are arranged on a frequency, and when modulation is performed at q = 3, there is a set of longitudinal modes whose phases are fixed every three. A set of longitudinal modes (ω ₁ , ω ₄ , ω ₇ ,.
ω _{3k + 1} ), a set of longitudinal modes indicated by dotted lines (ω ₂ , ω ₅ , ω
_{8, ...... ω 3k + 2)} , and longitudinal mode of the set (omega ₃ indicated by a _{dot-hatched, ω 6, ω 9, ......} ω 3k + 3) is the super mode.

Here, k = 0, 1, 2,..., And the number of longitudinal modes is limited by the band of the optical filter. These super modes oscillate individually and compete with each other between the super modes. At this time, if one super mode oscillates with only one super mode completely suppressing other super modes, q pulses having equal energy are formed in the resonator at equal intervals, and stable harmonic mode locking is performed. Is realized. However, if several super modes oscillate at the same time, the energy fluctuates between the pulses, and the pulses become unstable, resulting in super mode noise.

For example, f ₀ = 10 MHz, f = 10 GH
Figure 3 shows how super mode noise is generated when harmonic mode locking is performed with a conventional laser pulse oscillator.
Shown in FIG. 3 shows an electric spectrum. In addition to the component having the strongest intensity of 10 GHz, weak spectral components are arranged at a longitudinal mode interval. In this state, the energy of each pulse is not fixed, and there is a variation in energy between pulses.

As described above, in the conventional harmonic mode-locked laser pulse oscillator, although a high repetition optical pulse is generated, since there is no means for controlling the super mode noise, a stable light in which the energy of each pulse is constant is maintained. It was difficult to generate a pulse train.

An object of the present invention is to provide a laser pulse oscillator capable of stably generating a high repetition optical pulse.

[0015]

According to the present invention, there is provided a light source provided in a resonator of a laser at a frequency corresponding to a higher integer multiple of a fundamental frequency determined by the resonator length of the laser. In a laser pulse oscillator that drives a modulator to oscillate an optical pulse by harmonic mode locking, a means for generating a non-linear phase change due to a self-phase modulation effect in a resonator and limiting a spectrum width of the optical pulse Band limiting means is provided.

According to the above configuration, the optical pulse having a large peak intensity is given a non-linear phase change by the self-phase modulation effect, and the spectrum width is widened. The energy is reduced due to a loss greater than the gain in the vessel.
On the other hand, an optical pulse having a small peak intensity has a small non-linear phase change given by the self-phase modulation effect and a small spread of the spectrum width. As a result, energy is increased due to a loss smaller than the gain in the resonator. As a result, a loss depending on the peak intensity is given to each optical pulse, the energy becomes constant, and supermode noise is completely suppressed.

The modulation signal for driving the optical modulator includes a sine wave signal generated by a synthesizer outside the resonator,
Alternatively, a sine wave clock signal extracted from a part of the laser output can be used.

As means for generating the self-phase modulation effect, an optical fiber having a length capable of giving a nonlinear phase change of about π / 30 or more to an optical pulse can be used. Further, in this optical fiber, the use of the optical fiber made of a material having a large nonlinear refractive index than the optical fiber of quartz-based, can be shorter resonator length.

[0019]

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings.

FIG. 4 shows a first example of the laser pulse oscillator according to the present invention.
In the drawings, the same components as those of the conventional example are denoted by the same reference numerals. That is, 1 is a rare earth doped optical fiber, 2 is an excitation light source, 3 is an optical coupler, 4 is an optical splitter,
5 is an optical isolator, 6 is an optical modulator, 7 is an optical filter,
8 is a synthesizer, 9 is an amplifier, and 10 is an optical fiber.

Here, for example, when an erbium-doped optical fiber is used as the rare-earth-doped optical fiber 1, the laser oscillation wavelength is in the 1.5 μm band. As the excitation light source 2, a semiconductor laser can be used. As the light modulator 6, for example, a Mach-Zehnder type intensity modulator made of lithium niobate, an electroabsorption modulator, a semiconductor laser modulator, or the like can be used.

In the laser pulse oscillator shown in FIG. 4, when the rare earth-doped optical fiber 1 is excited by the excitation light source 2 through the optical coupler 3, continuous light oscillation occurs in the forward direction of the optical isolator 5 in the transmission band of the optical filter 7. . Next, when the modulation frequency of qf ₀ output from the synthesizer 8 is applied to the optical modulator 6 through the amplifier 9, an optical pulse train of qf ₀ is repeatedly generated by harmonic mode locking.

If the peak intensity of the light pulse generated during the mode locking process becomes strong, a nonlinear optical effect is induced in the optical fiber 10, and the self-phase modulation effect particularly easily occurs in proportion to the incident light intensity. The nonlinear phase change amount φ _NL due to the self-phase modulation effect is given by φ _NL = 2πn ₂ PL _eff / λA _eff (1) (GPAgrawal “Nonlinear Fiber Optic
s "Referring Academic Press, 1989). Here, n ₂ is the nonlinear refractive index of the optical fiber 10, P is the peak intensity of the optical pulse in the resonator, L _eff is the effective length of the optical fiber 10, lambda is the wavelength, A _eff is the effective area of the optical fiber 10.

The peak intensity P of the light pulse in the resonator
Is obtained as follows. That is, if the average intensity P _AV and the pulse width τ of the laser output taken out from the optical splitter 4 are measured, the peak intensity of the optical pulse when the repetition of the optical pulse is f is given by P _AV / (τf). Therefore, if this is multiplied by the reciprocal of the branching rate of the optical branching device 4, the peak intensity P of the optical pulse in the resonator can be obtained. Further, when L _eff is the loss of the optical fiber 10 to _{α, L eff = (1-} e -αL)
/ Α.

As can be seen from the above equation (1), when the loss of the optical fiber 10 is small, the peak intensity is high, and the longer the effective length of the optical fiber 10 is, the larger φ _NL , that is, the nonlinearity becomes. When the self-phase modulation effect occurs, the light pulse undergoes frequency modulation (chirp), and the spectrum spreads.

Next, suppression of super mode noise by a combination of the self-phase modulation effect and the band-pass type optical filter will be described. Here, a situation after an optical pulse train is formed by intensity modulation will be considered.

As the peak intensity of a light pulse increases,
Although the spectrum spreads due to the self-phase modulation effect, both ends of the spectrum are shifted when passing through a narrow-band optical filter (bandpass filter) having a fixed band, and a larger loss is caused. As a result, since the gain in the resonator is constant, the energy of the light pulse is reduced.

On the other hand, when the peak intensity of a certain optical pulse decreases, the spread of the spectrum due to the self-phase modulation effect decreases, and the spectrum tail can pass through the optical filter with lower loss. As a result, the energy of the light pulse increases.

That is, the light pulse receives a loss depending on the peak intensity in the resonator, and the energy increases when the intensity is lower than a certain peak intensity, and decreases when the intensity is higher than a certain peak intensity. By this action, the energy of the light pulse is fixed at a certain peak intensity.

Since the self-phase modulation effect occurs in femtoseconds, the nonlinear loss depending on the intensity has an extremely fast reaction speed and can sufficiently respond to individual pulses of high repetition of several tens of GHz. Therefore, by combining the non-linearity due to the self-phase modulation effect and the band-pass optical filter, the energy of each pulse is always constant, and the super mode noise is completely suppressed.

For example, in a conventional laser oscillator, φ
_{Since the NL} is as small as π / 100, the self-phase modulation effect cannot be sufficiently obtained, and the effect of keeping the energy of each optical pulse constant does not work. Therefore, super mode noise as shown in FIG. 3 is generated. Was. In the laser oscillator of the present invention, L
When the self-phase modulation effect is sufficiently generated so that φ _NL becomes π / 10 using the optical fiber 10 of = 200 m, the energy of each light pulse is increased by the combination of the nonlinearity due to the self-phase modulation effect and the optical filter. And the super mode noise is completely suppressed. Fig. 5
Shown in

The non-linearity due to the self-phase modulation effect required to suppress super mode noise is π / 30 or more.
When a silica-based optical fiber is used as the optical fiber 10, in order to stably generate an optical pulse train having a pulse width of 7 ps at a repetition of 10 GHz, considering the light intensity coupled to the optical fiber of a practical excitation light source, 40m long
It is necessary to do above.

When a chalcogenide fiber, for example, is used as a material having a higher nonlinear refractive index than that of a silica-based optical fiber, the resonator length can be shortened because nonlinearity due to the self-phase modulation effect easily occurs.

[0035]

FIG. 6 shows a second embodiment of the laser pulse oscillator according to the present invention.
In the figure, the same components as those in the first embodiment are denoted by the same reference numerals. That is, 1 is a rare earth doped optical fiber, 2 is an excitation light source, 3 is an optical coupler,
11 is an optical splitter, 5 is an optical isolator, 6 is an optical modulator,
7 is an optical filter, 9 is an amplifier, 10 is an optical fiber, 12
Is a clock extractor, and 13 is a phase shifter.

The output of the laser is taken out through the optical splitter 4 and further divided by the optical splitter 11, a part of which is received by a light receiving element.
The electric signal is input to a clock extractor 12 comprising a narrow band filter and an electric signal amplifier. Next, a clock signal of a sine wave of a specific frequency is extracted from the laser output by the clock extractor 12, and the phase is adjusted by the phase shifter 13,
After the clock signal is amplified by the amplifier 9, it is applied to the optical modulator 6. In the resonator, light is intensity-modulated at a frequency synchronized with the clock signal, but since this light is originally a signal emitted in the resonator, it is always optimally modulated.

Here, consider the case of using a 10 GHz clock extractor. 1 that does not match the integral multiple of the fundamental frequency
Since the clock signal near 0 GHz cannot generate a stable pulse train, it disappears during the clock extraction process.
In the case of a clock signal that matches the integral frequency of the fundamental frequency, the modulation frequency and the repetition of the optical pulse completely match, so that stable pulse oscillation is gradually strengthened. When this is repeated, the optical modulator 6 is driven by one clock signal near 10 GHz which is initially noise-like and coincides with an integral multiple of the fundamental frequency, and the 10 GHz harmonic mode locking is achieved. You.

In this configuration, even if the length of the resonator changes due to a temperature change or the like and the repetition of the optical pulse changes, the modulation is performed with the clock signal synchronized with the repetition of the optical pulse. There is no shift between repetitions. Therefore, there is a feature that the pulse oscillation is stably continued for a long time without deteriorating the waveform of the optical pulse due to the temperature fluctuation.

In this laser pulse oscillator,
By using the self-phase modulation effect and the band-pass optical filter in combination, the energy of each optical pulse is equalized and the supermode noise can be completely suppressed, as in the case of the first embodiment. .

[0041]

As described above, according to the present invention,
In a harmonic mode-locked laser pulse oscillator, it is possible to give to each optical pulse an ultrafast response loss depending on the light intensity created by a combination of a means for generating a self-phase modulation effect and a band limiting means, This makes it possible to equalize the energy of each optical pulse, completely suppress supermode noise, and generate a stable high-repetition-rate optical pulse train.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an example of a conventional laser pulse oscillator.

FIG. 2 is a conceptual diagram of a super mode.

FIG. 3 is a diagram showing an example of a spectrum in a conventional laser pulse oscillator.

FIG. 4 is a configuration diagram showing a first embodiment of a laser pulse oscillator according to the present invention.

FIG. 5 is an explanatory diagram showing an example of a spectrum in the laser pulse oscillator of the present invention.

FIG. 6 is a diagram showing a second embodiment of the laser pulse oscillator according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Rare earth doped optical fiber, 2 ... Excitation light source, 3 ... Optical coupler, 4,11 ... Optical splitter, 5 ... Optical isolator, 6 ... Optical modulator, 7 ... Optical filter, 8 ... Synthesizer, 9 ... Amplifier, 10 ... optical fiber, 12 ... clock extractor, 13 ...
Phase shifter.

──────────────────────────────────────────────────続 き Continued on the front page (56) References Nakazawa, M .; et al. , Electronics Letters, September 15, 1994, Vol. 19, pp. 1603 -1605. (58) Field surveyed (Int.Cl. ⁷ , DB name) H01S 3/00-3/30 G02F 1/365

Claims (3)

(57) [Claims] 1. An optical pulse oscillator driven by a harmonic mode locking by driving an optical modulator installed in a laser resonator at a frequency corresponding to a higher integer multiple of a fundamental frequency determined by a laser resonator length. A laser pulse oscillator provided with means for generating a non-linear phase change due to a self-phase modulation effect in a resonator and band limiting means for limiting the spectral width of an optical pulse. As a means to generate the phase modulation effect ,
The non linear phase variation with given El optical fiber, and silica-based
Composed of a material with a higher nonlinear refractive index than the optical fiber
A laser pulse oscillator using a fiber . 2. The laser pulse oscillator according to claim 1, wherein a sine wave signal generated by a synthesizer outside the resonator is used as a modulation signal for driving the optical modulator. 3. The laser pulse oscillator according to claim 1, wherein a sinusoidal clock signal extracted from a part of the laser output is used as a modulation signal for driving the optical modulator.