This invention relates to the field of quantum well lasers comprising a reflection means external to the laser cavity.
U.S. Pat. No. 5,715,263 issued to SDL describes an example of a laser shown in FIG. 2 of this patent comprising a quantum well laser 26 with an output mirror 27 outputting into an optical fiber 32. This type of laser is used in telecommunications to pump an amplifier outputting into a transmission line. According to the invention described in the SDL patent, the fiber 32 comprises a fiber Bragg grating 34 with the function of reflecting part of the light emitted by the laser 26 back to the laser 26. This patent (column 2, lines 37–45) describes how the optical spectrum of the emitting laser diode is affected if the center of the reflection band of the fiber Bragg grating is in the laser gain band. The exact effect depends on parameters such as the value of the reflection coefficient and band width of the fiber Bragg grating, the central wavelength of the grating with respect to the laser, the value of the optical distance between the laser and the grating, and the value of the current injected into the laser. In the SDL patent, the central wavelength of the Bragg grating is contained within a 10 nm band around the laser wavelength and the value of the reflection coefficient of the grating 34 is similar to the value of the output face 27 from laser 26. In the preferred embodiment, the width of the band reflected by the grating 34 and its reflection coefficient are such that the return into the laser cavity due to the output face is greater than the return due to the grating 34. Consequently, the grating 34 acts like a disturbance to the emission spectrum of laser diode 26, which has the effect of widening the emission band and thus making the diode less sensitive to disturbances caused by temperature changes or injected currents.
In order to obtain the required effect, in the preferred embodiment the grating 34 has a reflection peak that is located 1 or 2 nm from the wavelength of the diode, a reflection coefficient of 3% which, taking account of coupling between the grating and the diode, produces a return coefficient to the diode equal to 1.08%.
U.S. Pat. No. 5,563,732 issued to AT&T Corp. also describes a pumping laser 13 for an amplifier laser 12 also used to make optical transmissions. This laser 12 is stabilized to prevent fluctuations in the emitted wavelengths caused by parasite reflections from the amplifier laser 12 by means of a fiber grating 14. The inventors have found that the pumping laser 13 is stable if the reflection coefficient from the grating 14 is between 5 and 43 dB.
Experiments carried out by the applicant have shown that the use of lasers stabilized using a fiber grating can have a good influence on the operating stability of the laser and particularly on the stability of the emitted wavelength, but only within certain limits. In particular, the use of lasers stabilized as described in each of the two patents mentioned above cannot produce a laser capable of operating within a temperature range varying from −20° C. to +70° C. as currently required by most users. Therefore, there is a need for such a laser.
The invention relates to a quantum well laser like the lasers described in the two documents mentioned above, which is capable of operating without any particular precautions within a temperature range between two limiting temperatures defining a range of about 100° C., and particularly within the temperature range from −20° C. to +70° C. However, it should be understood that operating between −20° C. and +70° C. is not the same thing as widening the operating band in order to give a band with an output wavelength independent of reasonable fluctuations in the operating temperature, for example within a temperature range fluctuating by 5 to 6° about a nominal operating temperature.
As in the prior art, the invention uses a quantum well laser with a laser cavity formed by a laser medium between a reflection face and an output face with a reflection coefficient,
However, the invention is different from the prior art in one important respect. The inventors have observed that, at a given temperature, the gain curve for the cavity as a function of the wavelength, has a positive slope in the direction of increasing wavelengths, is maximum at a wavelength λmax, and then has a negative slope. The slope coefficient of the positive slope is much smaller than the slope coefficient after the maximum. By observing the manner in which the gain curve deforms as a function of the temperature, they found that, for example for a laser operating at 980 nm at 25° C., the maximum shifted between 966 nm at −20° C. and about 995 nm at 70° C. The displacement is approximately linear with a coefficient of about 0.3 nm per degree. For the system to operate over a wide temperature range, it is necessary that the condition under which the cavity gain is equal to cavity losses is satisfied for the wavelength of the fiber Bragg grating over the entire temperature range, despite deformations to the cavity gain curve as a function of the wavelength caused by temperature variations. The inventors found that this condition can be satisfied if the value of the reflection wavelength of the fiber grating at the median temperature is at least 10 nm less than the value of the wavelength λmax for which the cavity gain is maximum. In practice, the amount to be provided should be 15 plus or minus 5 nm. The fact of using a value of the wavelength equal to about 15 nm before this maximum means that the threshold condition at which the gain is equal to losses can be satisfied over a wide temperature range, at the grating wavelength.
In summary, the invention relates to an optical device comprising:
Preferably, the energy received by the laser cavity returning from the fiber grating is greater than the energy received in return through the laser output face.
This functional characterization may be clarified by a structural characterization defining a ratio relating the coefficients of the laser output face and the grating reflection coefficient. The product of the reflection coefficient for the fiber grating and the square of the loss coefficient due to coupling between the fiber and the laser must be greater than the reflection coefficient at the cavity output face. In this way, the energy received in return from the fiber grating can no longer be considered as being a disturbance widening the output optical spectrum. The value of the wavelength reflected by the grating determines the value of the laser output wavelength. In a known manner, the value of the wavelength λ reflected by the fiber grating varies with temperature much less than the cavity. The result is that with this configuration, the optical system formed by the laser, the fiber and the coupling means is capable of operating while remaining less dependent on local temperature variations. In one embodiment of the invention, the value of the grating reflection coefficient is more than ten times greater than the reflection coefficient from the laser output face.
An example embodiment of the invention will now be commented upon and explained using the attached drawings in which:
The optical focusing means are composed of a first collimation lens 3 followed by a focusing lens 4 that focuses light towards the center of the fiber 5, in a known manner.
The characteristic features of the invention will now be explained and commented upon in relation to the curves in
Obviously, the laser according to the invention may be used for the same purposes as described in prior art as mentioned above, and particularly to pump a power laser composed of a fiber doped with erbium.
Number | Date | Country | Kind |
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99 05528 | Apr 1999 | FR | national |
Number | Name | Date | Kind |
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5485481 | Ventrudo et al | Jan 1996 | A |
5563732 | Erdogan et al. | Oct 1996 | A |
5715263 | Ventrudo et al. | Feb 1998 | A |
5717711 | Doussierre ett al | Feb 1998 | A |
6233259 | Ventrudo et al. | May 2001 | B1 |
6240119 | Ventrudo | May 2001 | B1 |
6343088 | Mugino et al. | Jan 2002 | B1 |
Number | Date | Country |
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0 772 267 | May 1997 | EP |