JP3380749B2 - Fiber grating stabilized diode laser - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention emits intense light in a narrow bandwidth having a stable luminous intensity and wavelength suitable for optically exciting a laser such as a solid state fiber amplifier or a fiber laser. It relates to a stabilized laser source.

2. Description of the Related Art The background of the present invention will be described.
Optical fiber amplifiers and lasers are rapidly becoming important components of optical communication systems. Fiber optic amplifiers are used to enhance attenuated optical signals along fiber optic communication paths. In fiber optic communication systems, these have replaced the elusive electrical repeaters, enabling the realization of a true all-fiber optical communication system. Similarly, optical fiber lasers have been proposed for optical carrier generation for fiber optic communication systems. These lasers may be externally modulatable or mode-lockable, but in some cases may be used in place of diode lasers as powerful light sources in fiber optic communication systems. Both fiber amplifiers and lasers operate on similar principles. The silica glass in the waveguide portion of the optical fiber is doped with ions of a rare earth element such as erbium. The distribution of excited states of erbium ions is such that if the stimulated emission rate exceeds the spontaneous emission and absorption, the energy structure of erbium ions will amplify the signal light with a wavelength of about 1530-1565 nm in the fiber. . In such a situation, light within the gain bandwidth incident on the optical fiber will gain a net gain and exit from the fiber with increased power. For example, a mechanism that would recirculate this amplified signal through the fiber by placing appropriate reflectors at both ends of the fiber would result in a net gain equal to the loss of light within the optical bandwidth. Laser action occurs in the fiber. In any case, it is extremely important to excite erbium ions to bring them into an excited state suitable for generating a gain. This can be achieved by pumping erbium ions with light near 980 nm, but this light is most conveniently supplied by a powerful diode laser coupled to the waveguide portion of the optical fiber. . If the cross-sectional area of this part is relatively small, it can be ensured that the luminous intensity is high, so that a considerable gain of the signal wavelength can be expected. However, those skilled in the art will recognize that the characteristics of the signal generated by such an amplifier or laser
It will be appreciated that the nature of the diode laser used to excite the fiber itself will vary greatly. In a practical system, a diode laser is permanently and firmly connected to an undoped optical fiber using an opto-mechanical device, and then to an optical amplifier or laser doped fiber. An assembly consisting of a diode laser, an optomechanical device and an optical fiber is called a pigtail diode laser. Currently, many pigtailed diode lasers have undesirable characteristics, such as wavelength and luminous instability that can cause noise in pump-type systems. Wavelength 980 nm
The most troublesome sources of diode laser noise in these diode lasers are mode fluctuation noise and wavelength fluctuations caused by undesired variable optical feedback to the diode laser or changes in temperature or input current. This noise is not particularly preferable in a fiber amplifier because it increases the error of the amplified optical communication signal and lowers the practicality of the device. There are various techniques for reducing the influence of such diode laser noise. For example, an active electrical system that detects fluctuations in the output of a fiber amplifier caused by fluctuations in diode laser characteristics and feeds back a signal to the diode laser in the correct phase to reduce laser fluctuations. Unfortunately, this technique adds to the cost and complexity of the amplifier. It is preferable to use a passive method to reduce the oscillation of the diode laser. One attractive solution is to feed a portion of its own light back to the pump diode laser. These lasers are very responsive to optical feedback, and properly controlling such feedback can improve laser operation. Feedback is usually provided by an external reflector, such as a mirror or diffraction grating, and requires external optical elements, such as lenses, to manipulate and guide the light out of and back to the diode laser resonator. External optics and external reflectors are often quite compact, but adjusting such a system is difficult and expensive, and mechanical and thermal stability is also appropriate for use in fiber amplifiers and lasers. Often not. There is a need for more robust techniques for controlling the characteristics of diode lasers.

SUMMARY OF THE INVENTION An overview of the present invention is to provide a pigtailed diode laser using a Bragg fiber grating to provide optical feedback to the diode laser resonator, thereby reducing the frequency of the diode laser to a fiber. By fixing the frequency to the frequency of the diffraction grating, noise in the vertical direction due to mode fluctuation of the laser is reduced. A Bragg fiber grating is a periodic structure of refractive index changes in or near the guided mode portion of an optical fiber that reflects certain wavelengths of light propagating along the fiber. The reflected light propagates through the fiber in a direction opposite to the direction of the incident light. The diode laser is pigtailed to a fiber with a Bragg fiber grating, and affects the light spectrum of the diode laser if the center of the grating bandwidth is within the gain bandwidth of the laser. The extent of the effect depends on parameters such as the magnitude and bandwidth of the reflectivity of the diffraction grating, the center wavelength of the diffraction grating with respect to the laser, the distance between the laser and the diffraction grating, and the magnitude of the current supplied to the diode laser. You. In many cases, the properties of the laser can be improved by a given application. The device according to the invention comprises a diode laser, means for focusing the laser light emission on the optical fiber, and a diffraction grating formed in or near the waveguide of the optical fiber. In one of these aspects, the present invention comprises an apparatus for producing a stable laser source having a plurality of longitudinal modes with a very narrow bandwidth, the apparatus emitting light in a substantially single spatial mode. Consisting of a diode laser. An optical fiber is provided and a portion of the fiber is capable of guiding in at least one mode of a diode laser. Means is provided for directing light emitted from the diode laser to the optical fiber. The Bragg fiber grating is formed in the region of the guided mode portion of the optical fiber.

FIG. 1 shows a fiber amplifier 10 including a prior art pigtailed diode laser. The optical fiber 14 is doped with erbium (indicated by the numeral 16) to obtain an amplification effect and is connected to the undoped fiber 13. The amplification effect is obtained by exciting erbium atoms with light at approximately 980 nm. This uses an optoelectronic coupler 18 to pass a 980 nm
Achieved by coupling to the transmitted light from 1 (1550 nm). The 980 nm light source is a lens in the prior art
Provided by a pigtailed laser diode 20 comprising a laser diode 22 coupled to a fiber 24 undoped by 23. The limitations of prior art pigtailed laser diodes have been described above. FIG. 2 is a schematic diagram of a pigtailed diode laser according to the present invention. FIG. 3 is a graph comparing the spectra of the output of a prior art pigtailed diode laser and the pigtailed diode laser of the present invention. FIG. 4 is a schematic diagram of an embodiment combining the prior art fiber amplifier shown in FIG. 1 with the pigtailed diode laser of the present invention shown in FIG. FIG. 1 illustrates a fiber amplifier 10 including a prior art pigtailed diode laser. The optical fiber 14 is doped with erbium (indicated by the numeral 16) to provide an amplification effect, and the undoped fiber 1
Connected to 3. The amplification effect is obtained by exciting erbium atoms with light at approximately 980 nm. This is achieved by using an optoelectronic coupler 18
Transmission light from transmission fiber 15 (1550 nm)
Is achieved by combining In the prior art, a 980 nm light source is a laser diode 2 coupled to an undoped fiber 24 by a lens 23.
Provided by a pigtailed laser diode 20 composed of two. The limitations of prior art pigtailed laser diodes have been described above. FIG. 2 shows a pigtailed laser diode according to a preferred embodiment of the present invention. Diode laser 26 emits in a single spatial mode and is typically fabricated from an InGaAs semiconductor material with a quantum well epitaxial or index guided structure. Diode lasers are most conveniently excited by the application of current. Diode lasers with the required characteristics are commercially available. The diode laser 26 is configured to radiate mainly from a front output surface 27. The diffused laser diverging light 28 is directed by a focusing system 30 to the guided mode portion of an optical fiber 32, which has a Bragg fiber grating in the core. In the preferred embodiment focus adjustment system, the laser diode output is
The lens indicated by number 36 to focus on 2
System. Alternatively, the fiber can be located near the diode laser so that a substantial portion of the emitted light is collected by the fiber. The optical fiber 32 is typically made of quartz glass containing a trace amount of additives to improve the light guiding properties of the fiber. The fiber grating provides optical feedback to the diode laser. The fiber grating 34 can be made using lithographic techniques to etch near the guided mode portion of the fiber 34, or, more generally, to expose the fiber to a high fluence UV periodic light intensity variation pattern. When fabricating a diffraction grating using the latter technique, the core of the fiber should have a germanium concentration so that the core is more responsive to the ultraviolet light that forms the diffraction grating. Fiber 34 may maintain one or more spatial modes at the emission wavelength of the diode laser. The fiber grating 34 is selected such that the maximum reflectivity is within 10 nm of the emission wavelength of the diode laser, which is similar to the reflectivity of the exit surface of the diode laser. The reflectivity bandwidth of the grating is typically 0.05 nm to 2 nm. If the amount of optical feedback to the laser is above a certain magnitude, the diffraction grating
The system works well if 34 and the laser diode 26 are located several hundred micrometers to several kilometers apart. With such an arrangement, the diode laser will have improved improved characteristics suitable for pumping solid state amplifiers and lasers. The light captured by the fiber 34 normally propagates through the fiber endlessly, except as limited by the loss characteristics of the fiber. A Bragg fiber grating 34 is created in the guided mode or core of the fiber. The grating is made such that the wavelength of maximum reflection is within the gain bandwidth of the diode laser. The grating reflects a portion of the radiation of the diode laser back through the fiber and focusing system back to the diode laser. The remaining optical power travels through the fiber through the fiber grating.
The effect of a fiber grating on the characteristics of the optical output of a diode laser can be explained by considering the wavelength-dependent loss of a coupled resonator formed from the fiber grating. One skilled in the art will recognize that optical feedback from the fiber grating is effective in reducing the loss of light from the laser cavity within the bandwidth of the fiber grating. It is well known that lasers operate preferentially near the lowest loss wavelength. Accordingly, it is possible to shift the wavelength of the diode laser from the free-run value of the laser to the wavelength of the fiber grating. This shift occurs when the magnitude of the reflectance from the diffraction grating is sufficient.
This can occur if the wavelength of the fiber grating is within the gain bandwidth of the diode laser. The operation of a diode laser under conditions of optical feedback can be complicated by the effect of the laser resonator itself, when the laser resonator itself is formed at the end face of a semiconductor chip. Figure 4
Represents a fiber amplifier 10 including a pigtailed diode laser. Optical fiber 14 is doped with erbium to provide an amplifying effect, as described with reference to FIG. 1, and is coupled to undoped fiber 13. In this case, too, a light source of approximately 980 nm is transmitted through the transmission fiber 15 using the optoelectronic coupler 18.
By coupling to transmitted light (1530-1565 nm). The 980 nm light source is provided by a pigtailed laser diode comprising a laser diode 26 coupled to an undoped fiber 32 by a lens 36. In the present invention, it is important that the light source is stable, and under such circumstances, the fiber amplifier is effective. In the preferred embodiment of the present invention, the reflectivity and wavelength of the grating are selected such that the broadband feedback from the diode laser resonator is greater than the feedback from the fiber grating. In this situation, the feedback from the fiber grating acts as a perturbation of the coherent electric field formed in the diode laser cavity. This perturbation serves to break the coherence of the diode laser emission, thereby increasing the emission bandwidth by several orders of magnitude, resulting in a spectral distribution as shown by curve A in FIG. The Bragg fiber grating is effective in fixing the output of the diode resonator to the fixed wavelength of the diffraction grating and setting a plurality of longitudinal modes of the external resonator around the wavelength. The presence of the plurality of longitudinal modes reduces the mode fluctuation noise of the diode laser. This is called coherence decay of the diode laser. Furthermore, the center wavelength of the emission still remains near the wavelength of maximum reflection from the fiber grating. Since the diode laser is thus restricted to operate within the bandwidth of the diffraction grating, the wavelength of the diode laser does not significantly fluctuate due to changes in temperature or current. In addition, the laser is not perturbed by extraneous optical feedback from reflective components located beyond the fiber grating, but it does not allow the extraneous optical feedback to This is a case on the premise that it is smaller than the feedback. The diode laser according to the present invention does not have a single longitudinal mode transition of the laser resonator as seen in a free-run type diode laser. Such transitions cause the luminous intensity of the diode laser output to fluctuate significantly during the transition period due to competition between the two modes. Such a mode transition occurs due to, for example, a change in the input current or temperature of the laser, and impairs the operation of the optical amplifier or the fiber laser. The optical output of the present invention comprises more than 20 longitudinal modes of the external resonator. The distribution of the optical output between these modes may change, but the fluctuation of the laser luminous intensity is much smaller than that of a single mode, free-run type diode laser. The output power from the fiber tip of the diode laser system is only slightly affected by the presence of the grating in the fiber. With a weakly reflective grating, the output power from the fiber drops by approximately (1-R g). Here, Rg is the maximum reflectance of the diffraction grating. The injection current at the threshold of the laser is slightly reduced by the presence of the diffraction grating. This effect increases the output power from the fiber, thus canceling the aforementioned power reduction.
The scope of the present invention consists of systems in which the fiber grating is of arbitrary length from a diode laser. However, this length affects the operation of the diode laser. To ensure that the coherence decay of the laser emission is preserved, the fiber grating is located a sufficient optical distance from the front of the diode laser. This distance is
It must be much longer than the coherent length of the diode laser under the conditions of optical feedback described above. Doing so ensures that the optical feedback from the fiber grating maintains the incoherence and therefore the laser maintains coherence collapse. If the grating is placed within a few centimeters of the diode laser, the feedback from the fiber grating can be coherent with the electric field inside the laser cavity, so that the diode laser operates with a very narrow linewidth Will be. While such radiation is very useful in some applications, it is not stable for use in exciting fiber amplifiers and lasers.
This is because when the operating characteristics of the laser change, mode transition noise of the laser resonator is generated. In addition, the transition from coherence to incoherence operation of the diode laser is unavoidable, causing fluctuations in light intensity, which impairs the operation of the fiber optic amplifier and the laser.
The present invention uses an optical fiber having a low birefringence, which cannot maintain the polarity of the electromagnetic field in the guiding region when stress is applied. Thus, for example, when the fiber is bent and stressed, the polarization of the light propagating in the fiber and reflected back to the diode laser is in a different polarization state than that of the light emitted from the laser. Since diode lasers do not respond effectively to optical feedback of different polarizations, the characteristics of the diode laser system may not be optimal. However, if the length of the resonator is less than a few meters and the radius of curvature is greater than 10 cm, the properties of the laser system will generally still be sufficient to achieve the improvements described above. Therefore, in the present invention, it is not necessary to take measures for controlling the polarization of the optical feedback. In a preferred embodiment of the present invention, the stress deformation layer I
An nGaAs multi-quantum well diode laser is coupled to an optical fiber with a 60% efficient aspheric lens system. Efficiency is 60%. Lasers are typically 965-1025 n
Emit light at m. Fiber gratings have a high reflection bandwidth.
The maximum reflectivity at 0.2-0.3 nm is about 3%. Therefore, the effective reflectance Reff viewed from the fiber diffraction grating is generally as follows. Reff = ( ² Rg where (is the coupling efficiency of light from a single diode laser to the optical fiber and Rg is the maximum reflectivity of the fiber grating. For example, for this specific value, (0.6) ² (3%) =
1.08% holds. In contrast, the front of the diode laser has a nominal reflectance of 4%. This degree of optical feedback makes the best use of the available power since enough light can pass through the fiber grating and is sufficient to maintain the coherence breakdown of the diode laser. The wavelength of the reflectivity of the diffraction grating is nominally within 10 nm of the wavelength of the diode laser. The length of the diffraction grating is 1-2 mm. To ensure that the coherence decay of the laser radiation is maintained, the fiber grating is placed at least 50 cm from the front of the diode laser. If you want to keep the coherence of the laser system,
The fiber grating should be placed as close as possible to the emitting surface of the diode laser and should not be more than a few centimeters. The output power from the optical fiber in the preferred embodiment is reduced by up to several percent. A 150 Mw diode laser pigtailed with fiber with a 3% highest reflectivity fiber grating provides 90 Mw output power from the fiber within experimental uncertainty.
, Which is about the same as the output power from a fiber without a grating. FIG. 3 shows the spectrum of the optical output of the present invention. As can be seen from this figure, curve B is for 980 nm without the fiber grating.
4 is an output spectrum of an InGaAs pigtail diode laser. There is approximately 0.5% feedback from the broadband external reflector to the diode laser, which causes the laser wavelength to become unstable. In curve A, the diode laser is operating under the same conditions, but there is a fiber grating with a maximum reflectivity of 3% and a bandwidth of 0.3 nm. The improvement in output spectrum is evident. The output of the present invention is stable even when the input current or the temperature of the laser diode greatly changes. Thus, in some cases, temperature control of the laser diode is not required, which in turn eliminates the need for a laser cooler and associated control electronics. The power required to control the temperature of the laser is reduced accordingly.

Industrial Applicability As described above, the present invention provides a very stable and powerful light emission source, and the characteristics and characteristics of an optical amplifier and a laser which must be excited by using such a light emission source. It is now clear to improve stability. The preferred embodiment has been described in FIG. 4 for use with a fiber amplifier. This is for those skilled in the art.
It will be appreciated that a combination with a fiber laser can also be used.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a pigtailed diode laser connected to a fiber amplifier according to the prior art.

FIG. 2 is a schematic diagram of a pigtail type diode laser of the present invention.

FIG. 3 is a graph showing a comparison of the output spectrum of a prior art pigtailed diode laser with the pigtailed diode laser of the present invention.

FIG. 4 is a schematic diagram showing an embodiment of the present invention.

[Explanation of symbols]

10 Fiber amplifier 13 Optical fiber 14 Erbium-doped optical fiber 18 Coupler 26 Diode laser 27 Exit surface 28 Laser divergent light 30 Focus adjustment means 32 optical fiber 34 Bragg fiber grating 36 Lens means

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Rogers Grant Canada British Columbia Buoy 8X 1 Sea 9 Victory Wicklow Street 3248 (56) References JP-A-6-291423 (JP, A) Electronics Letters, 22 [3], p. 134-135 Electronics Letters, 21 [20], p. 933-934 Journal of Lightwave Technology, 4 [11], p. Applied Optics, 25 [9], pp. 1655-1661. 2421-2432 IEEE Journal of Quantum Electronics, 24 [12], pp. 224-242. 2441-2447 IEEE Journal of Quantum Electronics, 29 [9], p. 2421-2432 IEEE Journal of Quantum Electronics, 25 [6], p. 1143-1151 IEEE Photonics Technology Letters, 6 [8], p. 907-909 Bus, Single Frequency Semiconductor or Lasers, SPIE OPTICAL ENGINEERING P RESS, p. 81-90 (58) Field surveyed (Int. Cl. ⁷ , DB name) H01S 5/00-5/50 G02B 6/10 G02B 6/42 JICST file (JOIS)

Claims (7)

(57) [Claims] 1. A stable laser source having a plurality of longitudinal modes.
Device for generating substantially the same spatial mode
Emitting light, and providing a diode laser type resonator and the diode.
Output surface defining one end of the laser diode cavity (27)
Laser light in the longitudinal mode in the laser resonator
-Emitting diode laser (26) and said diode
An optical fiber (32) coupled to a laser resonator;
Optics to diode laser in (32) waveguide part of fiber
Bragg fiber diffraction with returnable reflection bandwidth
Grating (34) and light from said diode laser (26)
Focus for emitting laser light directly to fiber (32)
And adjusting means (30) for a plurality of vertical modes.
Operating within the bandwidth of the fiber grating
Inducing coherence decay in the laser
A mode that occurs when the iva diffraction grating (34) does not exist.
Optical fiber (32) guided so that fluctuation noise is reduced
A sufficient distance between the fiber grating and the output surface (27).
The Bragg-type fiber diffraction grating (34)
The wavelength from the optical cavity of the diode laser.
Bandwidth feedback is greater than feedback from fiber grating
An apparatus characterized in that it is selected as follows. 2. A stable laser source generating apparatus according to claim 1.
The fiber grating (34) has a maximum reflectivity
Is within 10nm of the emission wavelength of the diode laser
And the bandwidth of the reflectivity of the diffraction grating is 0.05
An apparatus characterized by having a thickness of 2 nm. 3. A stable laser source according to claim 1.
In the generator, the wavelength of the diode laser is
Shift from the free-run value to the wavelength of the fiber grating.
The maximum reflectivity from the fiber grating (34)
Is within the gain bandwidth of the diode laser (26).
An apparatus characterized in that: 4. A stable laser source generator according to claim 1.
When the light is a fiber (32) with low birefringence
Consisting of a diode laser (26)
aAs multiple quantum well diode laser
Lens means (36) serving as point adjusting means (30) is aspheric
Consisting of a lens system and a fiber grating (3
4) an optical element having a reflection band of 0.05 to 1 nm
With the device. 5. The method according to claim 1, wherein the plurality of longitudinal modes are stably provided.
In the generating device, the rare earth element (16) is doped
When using the fiber amplifier (14)
Noise and mode fluctuation noise generated by the operation of
To reduce fluctuations in noise and wavelength.
The (26) induces a laser of a plurality of longitudinal mode, coherency
With fiber grating (34) operating lens collapse
Noise caused by changes in the longitudinal mode
Suppressing the occurrence of noise, the fiber amplifier
Signal light having a wavelength of 0 to 1565 nm is transmitted into the fiber.
A device characterized by being amplified by: 6. The fiber amplifier according to claim 5, wherein
A fiber amplifier, wherein the diode laser (26) is a semiconductor diode laser of a Group 3-5 compound of the Chemical Periodic Table that emits a wavelength of about 980 nm. 7. A fiber amplifier (10) and erbium
Optical fiber (14) doped with
Fiber (13) and opto-electron
980nm light using Nix coupler (18)
The source is transmitted light (1530-1) from the transmission fiber (15).
565 nm).
An apparatus according to claim 5 or claim 6 comprising: