JP3363950B2 - Mode-locked laser device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mode-locked laser device for generating a pulse laser used for optical communication, optical measurement and the like.

[0002]

2. Description of the Related Art Conventional mode-locked laser devices are advantageous in that they are capable of generating ultrashort optical pulses (femtosecond range) and transform-limited optical pulses (optical pulses with a minimum time bandwidth). In particular, the mode-locked laser device having wavelength tunability is very promising for applications such as large-capacity, long-distance optical communication and high-speed optical measurement. (Reference S. Kawawanishi et al .: '100Gbit / s, 50km optical tra
nsmission employing all-optical multi / demultiplexi
ng and PLC timing extraction ', in Technical Digest
of OFC'93, No.PD2,1993, pp.13-16).

FIG. 2 is a block diagram showing an example of a conventional tunable mode-locked laser device (reference document RPDavey et.
al.:'High-speed mode-locked, tunable, integrated
Erbium fiber laser ', Electron. Lett., 1992,28, pp.4
82-484). In the figure, 101 is a light modulating means for modulating the loss or phase of light at a predetermined frequency, and 106 is a light modulating means.
101 is a drive power source, 102 is an optical amplifying means for amplifying the optical pulse modulated by the optical modulating means 101, 103 is an optical isolator for defining the traveling direction of the optical pulse and blocking reflected return light, 104 is the amplified optical pulse , 105 is an optical coupling means for optically coupling the above devices, and 108 is a wavelength tunable means for changing the transmission wavelength within the gain spectrum width of the optical amplification means 102.

The light modulation means 101 is mainly LiN.
A modulator using an electro-optic effect such as bO ₃ or an LD amplifier using a semiconductor or an electroabsorption modulator is used.

The optical amplifying means 102 is mainly Er or Nd.
Well-known rare earth-doped optical fiber amplifiers and semiconductor laser amplifiers to which rare earths such as are added are mainly used.

3A to 3C are configuration diagrams showing a rare earth-doped optical fiber amplifier, and FIGS. 3A to 3C show backward pumping, forward pumping, and bidirectional pumping, respectively. Shows. In FIG. 3, 501 is a rare-earth-doped optical fiber (hereinafter, referred to as RDF), 502 is a pumping light source that pumps RDF, 503 is a wavelength mixture that combines the pumping light from the pumping light source 502 and an optical pulse, and enters the RDF 501. It is a wave instrument.

FIG. 4 is a block diagram showing a semiconductor laser amplifier, in which 601 is a semiconductor laser amplifier (hereinafter referred to as an LD).
602 is an excitation current source of the LD amplifier 601.

As the optical coupling means 105, an optical fiber,
Channel type optical waveguide formed on a flat substrate (references
Y. Hibino et al.:'Silica-based optical waveguide ri
ng laser integrated with semiconductor laser ampli
fier on Si substrate ', Electron. Lett., 1992, 28,
pp.1932-1933) can be used.

The operating principle of the conventional mode-locked laser will be described with reference to FIGS. 5 (a) and 5 (b). Of FIG.
(a) is a figure showing the typical spectrum characteristic obtained by mode locking, and (b) of FIG. 5 is a figure which shows the time characteristic. In FIG. 2, an optical modulator 101, an optical amplifier 102,
Optical isolator 103, optical branching means 104, wavelength tunable means 10
8 are coupled in a ring shape via the optical coupling means 105 to form a ring resonator. Here, the optical path length R of the ring resonator, the physical length of the components of the ring resonator is L, and the refractive index is n, each of the physical length to the respective refractive index n _i (physical length ) L _i is the sum of the values (the respective optical path lengths), and R = Σn _i L _i (1) It is represented by the above equation (1).

Furthermore, in the ring resonator, a large number of longitudinal modes (fr = c / R:
c is the speed of light). Here, when optical modulation of the same repetition frequency fm as the longitudinal mode frequency interval is applied by the optical modulation means 101 in the ring resonator, the frequency fm is fm = fr = c / R (2) according to the above equation (2). expressed.

At this time, as shown in FIG. 5 (a), a mode-locked oscillation state in which the phases of all longitudinal modes of the frequency interval fr are aligned becomes, and as shown in FIG. 5 (b), the repetition period 1 /
An optical pulse train of fr is obtained. The pulse width corresponds to the reciprocal of the oscillation spectrum width Δν determined by the envelopes of many longitudinal mode spectra, and the center of this spectrum envelope is the central wavelength (frequency ν ₀ ). When the frequency fm is an integral multiple of the frequency fr, the harmonic mode-locking condition fm = Nfr = Nc / R (3) shown in the following equation (3) is satisfied, and the repetition cycle 1 An optical pulse train of / (N · fr) is obtained. Here, N is a natural number.

Generally, the gain spectrum width of the optical amplifier 102 used in this mode-locked laser device (wavelength band in which the gain of one round of the resonator is 1 or more) is wider than this oscillation spectrum width δν. For example, when an Er-doped fiber amplifier is used, the oscillation spectrum width δν is about 100 GHz (about 1 nm at wavelength) in the 1.5 μm band, while the gain spectrum width is 2 THz or more (about 20 nm at wavelength).
And above). The wavelength tunable means 108 is used to enable oscillation at any wavelength within the gain spectrum width of the wide band of the optical amplification means 102.

[0013] As the tunable device 108 is conventionally FIG
An optical bandpass filter composed of a dielectric multilayer film as shown in 6 is used. In the figure, 401 is a dielectric multilayer filter, 402 is an optical lens, and θ is an angle formed by a plane perpendicular to the traveling direction of light and the dielectric multilayer filter 401. When the incident angle θ on the dielectric multilayer filter 401 is changed, the transmission peak wavelength λp changes. Generally, a bandwidth of 0.5 to 3 nm and a variable wavelength width of about 50 nm are possible.
Therefore, by using the wavelength varying means 108 in the mode-locked laser device, the oscillation wavelength can be set to the transmission peak wavelength λp, and the oscillation wavelength can be changed within the gain spectrum width of the optical amplifying means 102.

[0014]

In the above-mentioned conventional mode-locked laser device, the oscillation wavelength can be selected by the wavelength tunable means 108. However, at each oscillation wavelength, the equation (2) or (3) is used. Must satisfy the mode synchronization conditions of. Also, when the oscillation wavelength changes, the optical path length R changes due to wavelength dispersion of the refractive index n of the resonator. As a result, the longitudinal mode frequency interval fr changes, so that it does not match the repetition frequency fm of the optical modulator,
It deviates from the mode synchronization condition. Therefore, in the above-described conventional device, the mode-locking condition is achieved by adjusting the repetition frequency fm by the drive power supply 106 so as to match the longitudinal mode frequency interval fr.

However, when the repetitive frequency fm changes, the repetitive frequency of the generated optical pulse changes, and it becomes impossible to synchronize with the outside. Therefore, it is applicable to the above-mentioned optical communication and optical measurement which require a desired repetitive frequency. It had the drawback of being difficult.

The present invention has been made in view of such conventional problems, and an object thereof is to provide a mode-locked laser device capable of changing the oscillation wavelength without changing the repetition frequency fm. And

[0017]

In order to achieve the above object, the present invention provides a mode-locked laser device according to claim 1, wherein the mode-locked laser device modulates a loss or a phase of light at a predetermined frequency, and a modulator. A ring including an optical amplification means for amplifying the generated optical pulse, an optical branching means for extracting the optical pulse to the outside, and an optical coupling means for optically coupling the respective means with each other to form a ring resonator. Type mode-locked laser device, by changing the transmission wavelength,
Wavelength / optical path length that controls the oscillation wavelength of the laser diode
The wavelength / optical path length changing means is provided with one or more changing means.
Is a single control signal for the transmission wavelength and the optical path length so that the total optical path length of the resonator does not change with respect to the change of the oscillation wavelength.
Therefore, it is characterized in that they are simultaneously controlled .

The mode-locked laser device according to a second aspect of the present invention includes an optical modulator that modulates the loss or phase of light at a predetermined frequency, an optical amplifier that amplifies the modulated optical pulse, and most of the incident light. Two light reflecting means for reflecting
A Fabry-Perot comprising: two Fabry-Perot type resonators which are arranged at both ends of the Fabry-Perot type resonator and optically couple with the optical modulator and the optical amplifier. in type mode-locked laser device, transmission
The mode-locked laser device by changing the wavelength
One or more wavelength / optical path length changing means for controlling the oscillation wavelength of
The wavelength / optical path length changing means includes a transmitted wave so that the total optical path length of the resonator does not change with respect to the change of the oscillation wavelength.
The length and the optical path length are controlled simultaneously by a single control signal .

[0019]

According to claim 1 of the present invention, a single control signal is obtained.
By providing the wavelength / optical path length changing means for changing the transmission wavelength and the optical path length at the same time in the ring resonator, the oscillation wavelength of the ring type mode-locked laser device can be controlled by the optical amplifying means without changing the repetition frequency. It can be varied within the gain spectral width.

According to claim 2, a single control signal
With the wavelength / optical path length changing means that changes the transmission wavelength and the optical path length at the same time in the Fabry-Perot resonator, the oscillation wavelength of the Fabry-Perot mode-locked laser device can be changed without changing the repetition frequency. It can be changed within the gain spectrum width of the optical amplification means.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram showing a ring type wavelength tunable mode-locked optical fiber laser device according to a first embodiment of the present invention. In the figure, the same components as those of the above-described conventional example are designated by the same reference numerals and the description thereof will be omitted. Further, the difference between the conventional example and the first embodiment is that a wavelength / optical path length changing means 107 is provided in place of the wavelength varying means 108 in the conventional example. That is, in FIG. 1, 101 to 106 are drive power sources for the optical modulating means, the optical amplifying means, the optical isolator, the optical branching means, the optical coupling means, and the optical modulating means, as in the conventional example. 107 is a wavelength / optical path length changing means capable of changing the transmission wavelength and the optical path length at the same time. As the wavelength / optical path length changing means 107, specifically, for example, an etalon type wavelength tunable filter using a liquid crystal or an electro-optical material (LiNbO ₃ etc.) can be used.

First, the etalon type wavelength tunable filter will be described with reference to FIG.

FIG. 7 is a block diagram showing an example of an etalon type wavelength tunable filter (reference document: Hirabayashi et al., “600 Channel Selectable Liquid Crystal Tunable Wavelength Filter”, Proc. -246). In FIG. 7, 201 is a liquid crystal or electro-optical material, 202 is a mirror,
203 is a transparent electrode and 204 is a glass plate. This etalon-type wavelength tunable filter is composed of two glass plates having a liquid crystal or electro-optic material 201, a mirror 202 and a transparent electrode 203.
It is a Fabry-Perot-Etalon type optical filter enclosed between 204. The transmission peak wavelength λp of this Fabry-Perot etalon type optical filter is determined by the optical length (= physical length × refractive index) between the two mirrors 202, and
It is expressed by equation (4). λp = 2n'L '/ m (4) where n'and L'are liquid crystal or electro-optic material, respectively.
It is the refractive index and physical length of 201, and m is a natural number.
Further, it is assumed that there is no phase shift of the light beam due to the two mirrors 202.

In the case of the etalon type wavelength tunable filter, the refractive index n'can be changed by applying the voltage V to the liquid crystal or the electro-optical material 201 by the transparent electrode 203, so that the voltage V is changed to the voltage V as shown in FIG. In response Fabry
The transmission peak wavelength λp of the Pero-etalon type optical filter can be changed. Typical band widths and variable wavelength widths are about 0.1 to 3 nm and about 140 nm, respectively. When an RDF amplifier or LD amplifier is used as the optical amplification means 102, any wavelength within the gain spectrum width can be selected. .

Next, the operation of the first embodiment will be described with reference to FIG. When the repetition frequency fm is fixed and the oscillation wavelength is changed, in order to satisfy the mode-locking condition,
The optical path length R of the resonator, which is the denominator of the equation (2) or (3), must be constant with respect to the wavelength change. In the present embodiment, the optical path length R = constant condition can be satisfied by disposing the wavelength / optical path length changing means 107 in the resonator as shown in FIG.

That is, assuming that the physical length and refractive index of the wavelength / optical path length changing means 107 are L ₁ and n _1, and the physical lengths and refractive indexes of the other resonators are L ₀ and n ₀ , R = constant. The condition can be rewritten as the following equation (5). R = n ₀ · L ₀ + n ₁ · L ₁ = constant (5) If the etalon type wavelength tunable filter described above is used as the wavelength / optical path length changing means 107, n ₁ · L _{1 is obtained} from the equation (4).
= Mλ / 2, the equation (5) can be rewritten as the following equation (6) R = n ₀ · L ₀ + mλ / 2 = constant (6).

Further, both sides of the equation (6) are differentiated by the wavelength λ, and the physical length L ₀ does not change depending on the wavelength (dL ₀ / d
Considering that λ = 0), the following equation (7) is derived. -M / (2L ₀ ) = dn ₀ / dλ (7) The formula (7) uses the etalon type wavelength tunable filter as the wavelength / optical path length changing means 107 shown in FIG.
If the value of (2L ₀ ) is matched with the chromatic dispersion dn ₀ / dλ of the refractive index n ₀ of the resonator other than the wavelength / optical path length changing means 107, the mode locking condition is automatically established with respect to the wavelength change. It means that.

That is, if the condition of the expression (7) is satisfied, the repetition frequency fm of the optical modulator is changed by simply changing the transmission wavelength of the etalon type wavelength tunable filter. It is possible to achieve wavelength-tunable mode-locked oscillation without changing the wavelength. For example, using an etalon-type wavelength tunable filter with m = 200, the wavelength dispersion value of the refractive index of the resonator is set to the value of a normal quartz optical waveguide (dn ₀ / dλ = -1 × 10 ⁴ (m ^-1 )). , An optical path length L using an LD amplifier as the optical amplification means and the optical modulation means
_{When 0} is set to 0.01 (m), equation (7) holds and tunable mode-locked oscillation becomes possible. In addition, most of the resonators have a dispersion flat fiber whose chromatic dispersion is as small as about 1/100 of that of ordinary resonators (Ref. LG Cohen et al., "Low-loss qua
druple-clad single-mode lightguides with dispersio
n below 2ps / km-nm over the 1.28 μm-1.65 μm wavelen
gth range, "Electron. Lett., vol. 18, pp. 1023-102
4, Nov. 1982), when m = 200, the optical path length L ₀ is about 1 (m) and RDF is used as an optical amplifying means.
Can also be used.

Further, this liquid crystal or electro-optical material 201
The method using is excellent in controllability because the refractive index n ₁ can be changed by the voltage V. Further, since there is no mechanically movable part, the optical delay means can be downsized and integrated with other components, which is suitable for downsizing of the device.

Next, a second embodiment of the present invention will be described.
FIG. 9 is a configuration diagram showing a Fabry-Perot type wavelength tunable mode-locked optical fiber laser device according to a second embodiment of the present invention. In the figure, the same components as those of the first embodiment described above are represented by the same reference numerals. That is, 101, 102, 105,
106 and 107 are optical modulation means, optical amplification means, and
The optical coupling means, the drive power source for the optical modulation means, and the wavelength / optical path length changing means have the same detailed configurations as those in the first embodiment. Further, 301 is a light reflecting means.
In the second embodiment, the optical modulation means 101, the optical amplification means
102 and wavelength / optical path length changing means 107 are two light reflecting means.
A Fabry-Perot type tunable mode-locked optical fiber laser device is constructed by coupling the optical coupling means 105 between 301.

That is, the second embodiment is the same as the first embodiment except that the resonator configuration is the Fabry-Perot type, and the oscillation wavelength is changed according to the same principle as the first embodiment. Is possible.

Therefore, if the etalon type wavelength tunable filter using the above-mentioned liquid crystal or electro-optical material (LiNbO ₃ etc.) is used as the wavelength / optical path length changing means 107, the refractive index n ₁ can be changed by the applied voltage V. Ru excellent controllability order to be.

Furthermore, since there is no mechanical moving part, optical delay
The means can be miniaturized and integrated with other components.
Yes, it is suitable for downsizing the device.

[0034]

As described above, according to the first aspect of the present invention.
Alternatively, according to the mode-locked laser device of claim 2, the oscillation wavelength of the mode-locked laser device is changed within the gain spectrum width of the optical amplification means without changing the repetition frequency of the optical modulator, that is, the repetition frequency of the output optical pulse train. Can be made. Furthermore, since the wavelength / optical path length changing means can be configured using liquid crystal or an electro-optical material, the wavelength and the optical path length can be electrically changed, so that it has excellent controllability and is suitable for downsizing of the device. ing.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a ring type wavelength tunable mode-locked optical fiber laser device which is Embodiment 1 of the present invention.

FIG. 2 is a block diagram showing a conventional wavelength tunable mode-locked optical fiber laser device.

FIG. 3 is a block diagram showing an example of a rare earth-doped optical fiber amplifier.

FIG. 4 is a configuration diagram showing an example of a semiconductor laser amplifier.

FIG. 5 is a diagram for explaining the operation principle of a conventional example.

FIG. 6 is a configuration diagram showing an example of a wavelength variable unit in a conventional example.

FIG. 7 is a configuration diagram showing an example of an etalon type wavelength tunable filter according to a first embodiment.

FIG. 8 is a diagram showing wavelength tunable characteristics of the etalon type tunable filter in the first embodiment.

FIG. 9 is a configuration diagram showing a second embodiment of the present invention.

[Explanation of symbols]

101 ... Optical modulation means, 102 ... Optical amplification means, 103 ... Optical isolator, 104 ... Optical branching means, 105 ... Optical coupling means, 106 ...
Driving power source, 107 ... Wavelength / optical path length changing means, 201 ... Liquid crystal (or electro-optical material), 202 ... Mirror, 203 ... Transparent electrode, 204 ... Glass plate, 301 ... Light reflecting means, 401 ... Dielectric multilayer film filter, 402 ... Optical lens, 501 ... Rare earth doped optical fiber (RDF), 502 ... Excitation light source, 503 ... Wavelength multiplexer, 601 ... LD amplifier, 602 ... Excitation current source.

Continuation of the front page (56) References JP-A-6-252482 (JP, A) JP-A-6-85366 (JP, A) Davey, R. et al. P. et al. , "High-speed mod e-locked, tunable, integrated erbium fibre laser", Electrics Letters, Vol. 28, No. 5, pp. 482-484. Shan, X .; et al. , "Stabilizing Er fibre soliton laser with pulse phase locking", Electronics Letters, Vol. 28, No. 2, pp. 182-184. (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 3/00-3/30

Claims (2)

(57) [Claims] 1. A light modulating means for modulating the loss or phase of light at a predetermined frequency, a light amplifying means for amplifying the modulated light pulse, a light branching means for taking out the light pulse to the outside, and each of the means. In a ring-type mode-locked laser device having an optical coupling means that optically couples each other to form a ring-type resonator, the wavelength controlling the oscillation wavelength of the mode-locked laser device by changing the transmission wavelength. -One or more optical path length changing means are provided, and the wavelength / optical path length changing means has a product of physical length and refractive index of m.
λ / 2 (where λ is the wavelength and m is a natural number) etalon type
A wavelength tunable filter, other than the wavelength / optical path length changing means of the mode-locked laser
When the physical length of the resonator was a L ₀ and refractive index n ₀
In addition, the mode-locked laser device is characterized in that the condition of -m / (2L ₀ ) = dn ₀ / dλ is satisfied . 2. A light modulating means for modulating a loss or phase of light at a predetermined frequency, a light amplifying means for amplifying a modulated light pulse, and two light reflecting means for reflecting most of incident light, Fabry-Perot type having two optical reflection means arranged at both ends, and the optical modulation means and the optical amplification means arranged between them to optically couple to form a Fabry-Perot type resonator. in mode-locked laser device, the mode one or more wherein the wavelength-optical path length changing unit wavelength-optical path length changing means for controlling the oscillation wavelength of the locked laser device by changing the transmission wavelength is, the physical length refraction The product of the rates is m
λ / 2 (where λ is the wavelength and m is a natural number) etalon type
A wavelength tunable filter, other than the wavelength / optical path length changing means of the mode-locked laser
When the physical length of the resonator was a L ₀ and refractive index n ₀
In addition, the mode-locked laser device is characterized in that the condition of -m / (2L ₀ ) = dn ₀ / dλ is satisfied .