JP2724278B2 - Optical pulse generator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse generator. More particularly, the present invention relates to harmonic mode-locked ring lasers.

[0002]

BACKGROUND OF THE INVENTION US patent application Ser. No. 07 / 835,813, filed Feb. 18, 1992, discloses the use of a harmonically mode-locked laser in an optical pulse source, particularly for soliton transmission. The advantage of using it as a possible pulse source is described. A soliton is an optical pulse having a specific shape, width and energy that corrects for chromatic dispersion effects in a single mode fiber due to specific non-linear effects in the fiber. Thus, each pulse can propagate over long distances in a single mode fiber while maintaining its shape and pulse width. Other advantages of soliton transmission include the ability to use an erbium amplifier, rather than an iterator, for amplifying the transmitted signal, and that soliton transmission allows wavelength division multiplexing and polarization division multiplexing, That is, the transmission line capacity can be further increased.

The harmonic mode-locked laser described in US patent application Ser. No. 07 / 835,813 uses an optical pump to excite an erbium fiber amplifier located in a closed loop resonator path. The optoelectronic modulator in the closed loop resonator path is driven at an appropriate frequency to generate a pulsed laser beam having a repetition rate that is harmonically related to the frequency spacing between adjacent resonant modes of the closed loop resonator path.
A feature of the invention described in US patent application Ser. No. 07 / 835,813 is that it has a Fabry-Perot optical cavity whose optical path has a free spectral range approximately equal to the pulse repetition rate of the light pulses of the ring laser. . Fabry-Perot resonators tend to equalize the energy, shape and width of the pulse taken as output. However, even with the invention described in US patent application Ser. No. 07 / 835,813, the harmonically mode-locked laser is clearly unstable in its pulsed output.

[0004]

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a harmonically mode-locked laser capable of producing a stable pulsed output.

[0005]

SUMMARY OF THE INVENTION US patent application Ser. No. 07/8
It has been determined that the instability of the harmonically mode-locked laser described in U.S. Pat. No. 5,813,13 is caused by small changes in the length of the closed loop caused by temperature fluctuations. In order to solve this instability, in the present invention, the first
And extract a second light beam. This first and second
Are respectively directed from first and second optical filters having optical passbands that deviate from the frequency but intersect the substantially center operating frequency of the laser.

As will be described in detail below, the free spectral range of a Fabry-Perot optical resonator is sufficient to permit a change in optical path length, manifested as a change in wavelength of light propagating along the optical path. Deviating from a frequency that is exactly equal to the pulse repetition rate by a different frequency. A device for detecting the difference in intensity of light passing through the first and second optical filters is used to generate a signal proportional to such difference in light intensity to adjust the optical path length, thereby producing a pseudo temperature-induced optical path length variation. Can be corrected.

According to one embodiment of the present invention, the correction of the optical path length is performed by passing a current proportional to the intensity difference through a resistor surrounding a part of the optical fiber in the optical path. The temperature is changed using the current change, and the optical path length of the optical fiber is changed, thereby correcting the pseudo optical path length change. This feature of the invention can also be used in devices other than harmonically mode-locked lasers, such as phase shifters, which must make small but accurately determined optical path length corrections.

[0008]

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

Referring to FIG. A part of the closed loop optical transmission line is defined by the single mode optical fiber 12.
"Single mode" refers to the ability of a fiber to transmit light in a single transverse mode only. Fibers support many longitudinal modes. Optical coupler 13 directs light energy from pump laser 14 to erbium fiber amplifier 15. Erbium fiber amplifiers are available, for example, from the Journal of Lightwave Technology.
Lightwave Technology), Vol. LT-4, No. 7 (July 1986), pp. 870-875, "Formation of low-loss optical fibers containing rare earth ions" by SBPoole et al. An active laser medium of the type disclosed in the article entitled "Processing and Features," which amplifies light along an optical fiber path when properly pumped by energy from the pump laser 14. It consists of an optical fiber doped with erbium. Various other types of active laser media can also be used to amplify light with a harmonically mode-locked ring laser. Optical energy propagates around a closed loop defined by the optical transmission path. The polarization controller 17 is used to correct for polarization variations in the ring.

[0010] The modulator 18 modulates the intensity of the coherent light and changes the coherent light into a series of pulses.
For example, the modulator may be a known Mach-Zehnder lithium niobate modulation, typically driven at a frequency in the gigahertz range by an electronic oscillator 19 to produce light pulses having a corresponding repetition rate. It is a vessel. A period in which the pulse repetition rate is approximately equal to the transition time of the light pulse around the light path divided by an integer, or the following equation: T = t / N (1) (where T is the light pulse period and t Is the transition time of the light pulse around the closed loop path, and N is an integer).

Referring to FIG. The closed loop has a series of resonance frequency modes M. The pulse repetition rate f ₂ is generated according to the equation (1). Pulse repetition rate f ₂ is generally obtained by the following equation. f ₂ = Nf ₁ (2) (where f ₁ is the frequency gap between adjacent resonant ring modes M, such as the gap between M ₁ and M ₂ )

The frequency f ₁ is obtained by the following equation. f ₁ = c / nl (3) where c is the speed of light, n is the index of refraction of the optical fiber, and l is the length of the closed loop path or the length of the laser ring.

[0013] As described in US patent application Ser. No. 07 / 835,813, a portion of the laser ring comprises:
It is defined by a Fabry-Perot resonator 24 composed of half mirrors 25 and 26, and functions to equalize the amplification degree of the output pulse. A part of the pulse train circulating in the ring is removed by the output coupler 27 and transmitted as an output pulse as shown in FIG. These pulses closely resemble hyperbolic secant squared pulses to be used as solitons for transmission over single mode optical fibers, providing the known advantages of soliton transmission of information. An isolator 28 is arranged in the ring,
As described above, the light propagates through the ring in the counterclockwise direction.

The Fabry-Perot resonator has a large number of equally spaced resonance modes R ₁ . . . R _n and define the frequency spacing of two adjacent modes (eg, R ₂
And the center-to-center spacing of R ₃ ). FSR can also be obtained by the following equation. FSR = c / 2d (4) (where, c is the speed of light in the resonator, and d is the optical path length between the opposing mirrors 25 and 26)

According to US patent application Ser. No. 07 / 835,813, the resonator 24 has an FSR that is approximately equal to the pulse repetition rate f ₂ of the ring laser or that satisfies the relationship FSR = f ₂ (5). It is configured to have.
If this condition is met, the Fabry-Perot resonator equalizes the energy carried by the continuous pulse, thereby equalizing the pulse amplitude of the generated pulse train. The ring laser mode M which most nearly coincides with the resonance peak of the resonator mode R emits laser light while suppressing the competitive mode. Equations (4) and (5) above describe the resonator 24
It is shown that the round trip time of the light within is equal to the desired pulse time.

The Fabry-Perot mode envelope 30 indicates the passband of the Fabry-Perot resonator 24 having mode R ₄ at the center frequency. The ordinate in FIG. 2 is a unit of measure for the fraction of light intensity passed by the resonator at various frequencies.

The addition of an internal Fabry-Perot optical cavity stabilizes the operation of the ring laser by suppressing undesirable modes of the harmonically mode-locked ring laser, but the laser may still sometimes exhibit instability. FIG. 3 illustrates one cause of such instability. When the length of the ring is about 20 m, the length of the ring changes depending on the wavelength even if the temperature changes by only 0.005 ° C. Thus, modes M ₁ and M ₂ are shifted in frequency and exhibit fairly uniform losses in the Fabry-Perot cavity. Both of these modes stimulate laser operation and produce mode beating, which results in amplified variations in the output pulse.

In accordance with the present invention, a Fabry-Perot resonator 24 is provided to generate an error signal that can be used to compensate for small changes in the length of the closed loop ring of the laser.
Slightly detune or deviate from its optimal frequency. Equations (2) and (5) above indicate that the FSR is
Indicates that the gap distance between them must be equal to an integer. If this is true, one of the ring modes M is located at the center of each Fabry-Perot mode R, as shown in FIG. In FIG. 4, the Fabry-Perot resonator is detuned such that the FSR shifts from a frequency exactly equal to the pulse repetition rate to a frequency equal to df.

For simplicity, FIG. 4 shows a ring mode corresponding to M ₂ in FIG. 2 (various Fabry-Perot modes R in FIG. 4).
Inside) are shown. Detuning the Fabry-Perot resonator can reveal small changes in the length of the ring as detectable large changes in the wavelength or frequency of the output light. The reason will be described with reference to FIG. Small length changes, moving the ring mode M ₂ of the Fabry-Perot modes R ₄ slightly right, transmission strength is greatly reduced at that frequency, as long as the change is the Fabry-Perot mode R
₃ corresponding ring mode M ₂ is moved to the maximum output region. Consequently, a small change significantly increases the strength of the frequency defined by the mode R _3, and the mode R
_4. Reduce the intensity of the frequency defined by ₄ . As proof, change of ring length to move the M ₂ in the opposite direction,
Dominant frequency shift is achieved by Fabry-Perot resonance mode R ₅
For the frequency defined by Therefore,
In FIG. 5, when M ₂ moves to the right in resonator R ₄ , the light intensity of mode R ₃ increases, and when M ₂ moves to the left in R ₄ , the light intensity of R ₅ becomes the other. Increase for Fabry-Perot mode R. Thereby, a minute change in the optical path length is clearly indicated as a detectable change in the output frequency.

Referring back to FIG. An apparatus 32 is shown for detecting frequency changes in light transmitted within the optical ring and for automatically making length adjustments to the ring in response to such frequency fluctuations. A beam splitter is used to extract two light beams 33 and 34 from the optical path. Beam portion 35 forms part of the optical path of the ring. Beam portions 33, 34 and 35 are all oriented through wedge-shaped etalon 37. The etalon is a tapered quartz body as shown.
Here, beam 33 is transmitted through a relatively thin portion of the etalon, and beam 34 is transmitted through a relatively thick portion. Using a wedge-shaped etalon and moving the etalon up and down as indicated by the arrows, 3
The frequency of the etalon transmission peak corresponding to all beams of the book can be changed simultaneously and at the same speed.

Referring to FIG. The etalon constitutes an optical filter for each of the three beams, with curve 39 indicating the optical passband of beam 35, curve 40 indicating the passband of light beam 34, and curve 41 indicating the passband of beam 33. Moving the etalon up or down, the center frequency f _c of the pass band 39 is moved to the right or left. A tapered etalon is constant, when the beam 33 and 34 are symmetrically arranged around the beam 35, as the frequency f _c is shifted, curves 40 and 41 are curves 39
Will be placed symmetrically around the The intersection of passbands 40 and 41 at frequency fc is _{equal to} passband 4
0 and 41 means that is symmetrical with respect to the frequency f _c.

The intensity of the beams 33 and 34 is
And 40. These detectors are conventional photodetectors that convert light intensity into electrical signal amplitude.
Since the passbands 40 and 41 are frequency dependent, changes in optical frequency are manifested by changes in light intensity detected by detectors 39 and 40. The output of the differential amplifier is input to the length adjusting device 43. Length adjuster 43 adjusts the length of the ring laser beam path as a function of such output.

Referring to FIG. The length adjusting device 43 includes a piezoelectric stack 45 coupled to a corner reflector 46.
Consists of Light 12 from the optical path is directed to mirror 47, corner reflector 46 and mirror 48, and
Return to optical path 12. The voltage from the differential amplifier 42 in one direction pushes the corner reflectors toward the mirrors 47 and 48, thereby shortening the optical path, while the voltage in the other direction is increased by an amount proportional to the voltage from the differential amplifier. Increase the light path. Needless to say, the optical path length adjustment is performed, for example, to compensate for a pseudo length change due to thermal action. The various parameters used can be determined mathematically, but it is more practical to determine them experimentally. That is, it is generally more practical to experimentally calibrate the necessary length adjustments for different inputs from the differential amplifier 42.

As is apparent from the above description, the detuning of the Fabry-Perot resonator 24 generates the differential voltage from the differential amplifier 42 necessary to compensate for pseudo length changes. Extremely effective. That is, as shown in FIG. 5, small length change generates a large shift in the intensity of the resonator mode R ₄ to R _3, then, the large shift length by the device 43 (see FIG. 1) A significant differential voltage can be generated to make the adjustment. FIG.
Without the detuning shown in FIG. 3, the length change specified in FIG. 3 by the shift in the ring mode M would not be expected to produce a significant differential voltage for which the length adjustment could be made. Although the detuning frequency df in FIG. 4 can be determined mathematically, it is much easier to determine it experimentally. That is, the detuning frequency df is must be large enough to cause mode shift shown in FIG. 6, not at the same time, be less than the frequency gap f ₁ of the adjacent ring modes M of FIG.

FIG. 8 shows another device which can be used as the length adjuster 43 of FIG. This device comprises a portion of a single mode optical fiber 12. As shown, this single mode optical fiber 12 comprises a core layer 50 within a cladding layer 51. Around the cladding layer is a resistance layer 52,
Conduct current as indicated by the arrow. Resistive layer 52 is, for example, tin or a similar resistive metal having a thickness of about 20 microns, through which current is conducted during operation. The voltage from the differential amplifier 42 reduces the current through the resistance layer to lower the temperature of the optical fiber, or
The current is increased by the resistance layer to increase the temperature of the optical fiber. The change in temperature slightly changes the length of the optical fiber portion surrounded by the resistance layer 52, and performs necessary length adjustment.

The length adjuster of FIG. 8 is generally preferred over the length adjuster of FIG. The coating 52 can be formed very accurately, and the length change of the optical fiber can be controlled with extremely high precision. Temperature is 0.006 ℃
When changed, the 15 meter long fiber optic section can be shifted by about one wavelength. Therefore, according to the length adjuster in FIG. 8 than in the length adjuster in FIG. 7, a larger and more accurate length change can be obtained.

Due to this advantage, the device of FIG. 8 is also useful in circuits other than the circuit shown in FIG. For example, FIG.
Can be used as an optical phase shifter. FIG.
See If the optical pulse trains from the transmission lines 54 and 55 are to be multiplexed, it is desirable to adjust the relative phase of the optical pulse trains before multiplexing by the multiplexer 56. To this end, the phase of the pulse on line 55 can be finely adjusted using a phase shifter 57 on one of the lines. The device of FIG. 8 is best suited for this purpose. This is because the fine adjustment of the current conducted from the resistive coating 52 fine-tunes the effective length of the optical fiber portion, and consequently fine-tunes the phase shift generated by the fiber portion. The multiplexer interleaves the pulses from the two transmission paths. And for this reason, it is important to be able to control the relative phases of the two sets of pulses. The phase shifter can also be used for the purpose of adjusting the phase of an optical clock circuit, as is useful in many optical circuits. The phase shifter is
Analog circuits as well as circuits using pulse signals are useful.

As described in US patent application Ser. No. 07 / 835,813, ring lasers embodying the present invention generally operate at a wavelength of 1.555 microns. A laser output of 100 milliwatts at a wavelength of 1.48 microns can be applied. The total length of the closed loop ring is 20 meters. The modulator 18 can operate at 2.5 GHz. The frequency gap between ring modes M in FIG. 2 is typically 7 megahertz, and the FSR or center-to-center spacing for Fabry-Perot mode R is 2.5 gigahertz. The width of each Fabry-Perot mode R is 16
Megahertz, and the detuning frequency in FIG. 4 is 100 kilohertz. The bandwidth of each of the wedge-shaped etalon sections shown in FIG. 6 is about 60 GHz. Various other parameters of the circuit of FIG. 1 are described in U.S. patent application Ser. No. 07 / 835,813.

[0029]

As described above, according to the present invention,
A harmonically mode-locked laser that can generate a stable pulsed output is obtained.

[Brief description of the drawings]

FIG. 1 is a schematic circuit diagram of a harmonic mode-locked laser according to a representative embodiment of the present invention.

2 is a waveform diagram of various resonance modes of the Fabry-Perot resonator and the closed loop ring of FIG. 1;

3 is a waveform diagram of various resonance modes of the Fabry-Perot resonator and the closed loop ring of FIG. 1;

FIG. 4 is a waveform diagram of various resonance modes of the Fabry-Perot resonator and the closed loop ring of FIG. 1;

5 is a waveform diagram of various resonance modes of the Fabry-Perot resonator and the closed loop ring of FIG. 1;

FIG. 6 is a waveform diagram showing a pass band of a filter portion of the device of FIG. 1;

FIG. 7 is a schematic configuration diagram of an example of a length adjusting device that can be used in the circuit of FIG. 1;

FIG. 8 is a schematic configuration diagram of another example of the length adjusting device that can be used in the circuit of FIG. 1;

FIG. 9 is a schematic configuration diagram of a phase shifter according to another embodiment of the present invention.

[Explanation of symbols]

 Reference Signs List 12 single mode optical fiber 13 optical coupler 14 pump laser 15 erbium amplifier 17 polarization controller 18 modulator 19 electronic oscillator 24 Fabry-Perot resonator 25, 26 half mirror 27 optical coupler 28 isolator 30 envelope 32 length adjusting device 33, 34 , 35 beam 37 wedge-shaped etalon 39, 40 detector 42 differential amplifier 43 length adjuster 45 piezoelectric element 46 corner reflector 50 core layer 51 clad layer 52 resistance layer 54, 55 transmission line 56 multiplexer 57 phase shifter

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Lynn Frederick Mornool United States 07722 New Jersey Manmouth County Colts Neck, Carriage Hill Drive 11

Claims (10)

(57) [Claims] A means for defining an optical transmission path; an active laser medium included in the transmission path; and means for causing the active laser medium to emit coherent light having a predetermined frequency. Means (18) for propagating the emitted light as continuous pulses having a substantially uniform pulse repetition rate at the sublight frequency along the transmission line; and wherein the length of the transmission line is equal to the period of the pulse repetition rate. An optical resonator comprising: means for removing a part of the pulse from the optical transmission line, wherein the optical transmission line has an equispaced resonance.
Adjoining resonances are separated by a frequency that is approximately equal to the pulse repetition rate, extract a first light beam (33) and a second light beam (34) from the optical transmission line, and are replaced by a frequency;
Through the first and second optical filter (37) having an optical passband that intersect at a frequency f _c, respectively, means are provided for orienting the first and second light beams, the first and second light An optical pulse generator provided with means (42, 43) for using the intensity difference to detect the intensity difference of the light passing through the filter and to adjust the length of the optical transmission line; . 2. The optical pulse generator according to claim 1, wherein said optical transmission line is a closed loop, and a main part of said optical transmission line is defined by a single mode optical fiber. 3. The optical pulse generator according to claim 1, wherein said optical resonator is a Fabry-Perot resonator. 4. The optical transmission line has a plurality of resonance modes, each mode being separated by a frequency f ₁ , wherein the Fabry-Perot resonator has a frequency less than f ₁ from a frequency equal to a pulse repetition rate. 4. An optical pulse generator according to claim 3, having a free spectral range shifted by frequency. 5. The first and second filters comprise a single wedge-shaped etalon, and means (45) are provided for moving the etalon so that the etalon crosses an optical transmission line. The optical pulse generator according to claim 1, further comprising: 6. The light pulse generator of claim 5, wherein the first and second light beams and the light transmission path all intersect the wedge-shaped etalon along substantially parallel paths. 7. The means for adjusting the length of the transmission path includes a photodetector (39, 39) in the path of the first and second beams.
40), wherein the output of the photodetector is input to a differential amplifier (42), and the output of the differential amplifier is input to a device (43) for adjusting the length of the optical transmission line. 2. The optical pulse generator according to claim 1, wherein: 8. The optical transmission line defining means comprises an optical fiber, and the optical transmission line length adjusting means (43) selectively resistance-heats a portion (12) of the optical fiber. 52. The optical pulse generator according to claim 1, comprising: 9. A portion of said optical fiber comprises a single mode optical fiber coated with a resistive material coating, and said selective heating means (52) comprises means for passing a current through a resistive metal coating. 9. The method according to claim 8, wherein
Light pulse generator. 10. The means for propagating light as a continuous pulse comprises an optical modulator for modulating optical transmission in an optical transmission line at a rate equal to a pulse repetition rate;
The optical pulse generator of claim 1, comprising a closed loop having a transit time approximately equal to an integer value of the pulse repetition rate period.