JP2005183426A - External resonance type semiconductor laser and semiconductor laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an external resonance type semiconductor laser in which even if output power or an operating temperature changes, the wavelength of an emitted laser beam changes only within a predetermined range. <P>SOLUTION: The external resonance type semiconductor laser is constituted of a semiconductor laser, a collimator lens, a supporting member, and a grating. The semiconductor laser includes, for example, a semiconductor laser element such as a laser diode for emitting the laser beam. The length L of the semiconductor laser element in the laser beam emitting direction is determined by L=(λ<SP>2</SP>)/(2×n×▵λ) when the predetermined wavelength of the laser beam emitted from the semiconductor laser element is λ, the allowable width of the wavelength of the laser beam is ▵λ, and the refraction index of the semiconductor laser element is n. Since the length L is determined as explained above, the wavelength of the laser beam emitted from the external resonance type semiconductor laser changes periodically within a range of ▵λ even if output power or an operating temperature rises. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an external resonator type semiconductor laser including a laser diode (LD).

  Laser light emitted from a semiconductor laser element such as a laser diode is originally a multimode, but by configuring an external resonator type semiconductor laser by combining a semiconductor laser and an external resonator, a constant wavelength can be obtained. Only light can be emitted, that is, a single mode can be achieved. This external resonator type semiconductor laser stabilizes the wavelength of the oscillation light of the semiconductor laser by making light of a predetermined wavelength incident on the semiconductor laser from the outside.

  Typical examples of the external resonator type semiconductor laser include a Littrow type semiconductor laser and a Littman type semiconductor laser. Here, for the Littrow type external cavity semiconductor laser 50, FIG. Will be described with reference to FIG. For example, longitudinal multimode laser light (oscillation light) emitted from a semiconductor laser 55 including a semiconductor laser element such as a laser diode is collimated by a lens (collimating lens) 56 and is incident on a grating (diffraction grating) 54. Is done. The grating 54 outputs first-order diffracted light of each mode, and specific first-order diffracted light is back-injected into the semiconductor laser 55 via the lens 56 according to the arrangement angle. As a result, the semiconductor laser 55 resonates with the injected first-order diffracted light and emits single-mode light, and the wavelength of the light is the same as the wavelength of the first-order diffracted light returned from the grating 54. . Furthermore, the remainder of the laser light (zero-order light) incident on the grating 54 is reflected at the same angle as the incident angle and emitted to the outside.

  The wavelength of light incident on the semiconductor laser 55 can be adjusted by changing the arrangement angle of the grating 54. The grating 54 is held by a leaf spring 52 supported by the first support portion 53. When the screw 51 in contact with the grating 54 is rotated, the grating 54 moves in the direction of the lens 56 or in the opposite direction. . This movement approximates a movement in which the grating 54 rotates around a place where the plate spring 52 is supported by the first support portion 53, and the rotation angle changes the arrangement angle of the grating 54.

  The grating 54 is normally a reflection type and is arranged at an angle of about 45 ° with respect to the laser light emitted from the semiconductor laser 55. Accordingly, the laser light emitted from the semiconductor laser 55 is reflected by the grating 54 with a bend of about 90 °. The semiconductor laser 55 and the lens 56 are supported by the second support part 57 so as to maintain a predetermined interval.

  Non-Patent Document 1 discloses a Littrow type external cavity semiconductor laser having a similar structure.

L. Ricci, et al .: "A compact grating-stabilized diode laser system for atomic physics", Optics Communications, 117 1995, pp541-549

  Further, the wavelength of the laser light emitted from the semiconductor laser 55 has a property that when the output power changes, it slightly changes according to the change. This is because the operating temperature of the laser diode (laser chip) increases as the output power increases. If there is no external resonator, the laser emitted from the semiconductor laser as shown in FIG. The center wavelength of the light is linearly shifted to the longer wavelength side as the output power (or operating temperature) increases.

  In such a case, when it is desired to change the output power while keeping the center wavelength constant, an element for adjusting the output power of the semiconductor laser is arranged so as to keep the wavelength constant. Such an element for adjusting the output power is, for example, a filter.

  On the other hand, some applications using laser light can tolerate some wavelength fluctuations in a single mode. The case of using for recording in a holographic memory is one such example. In this application, the laser light once divided into two is superimposed again on the hologram (recording medium), and the diffraction grating is recorded on the hologram by utilizing the coherence. Data is recorded by recording the diffraction grating.

  However, even when laser light is applied to recording in a holographic memory, only a slight wavelength variation is allowed, and the laser light exceeds the allowable range of wavelengths required according to the recording method and recording density. It is necessary to control so that it does not fluctuate. Further, since the wavelength control as described above uses a filter or the like, the number of parts increases and the circuit becomes complicated.

  Accordingly, an object of the present invention is to provide an external cavity semiconductor laser in which the wavelength of emitted laser light varies only within a predetermined range even when the output power or the operating temperature changes.

  Furthermore, an object of the present invention is to provide an external resonator type semiconductor laser whose wavelength varies within the predetermined range without adding elements or arranging complicated mechanisms.

  The present invention includes a semiconductor laser that emits laser light, a collimating lens that emits laser light received from the semiconductor laser as substantially parallel light, and a grating that receives the laser light via the collimating lens. The first-order diffracted light from the laser beam is incident on the semiconductor laser through the collimator lens, so that the single mode is realized. The semiconductor laser includes a semiconductor laser element that emits laser light, and the semiconductor in the laser light emitting direction The length of the laser element is L, where L is the desired wavelength of the laser light emitted from the semiconductor laser element, λ is the allowable width of the wavelength of the laser light, and the refractive index of the semiconductor laser element Is an external cavity semiconductor laser determined by L ≧ (λ ^ 2) / (2 × n × Δλ) when n is n.

  The present invention also relates to a semiconductor laser that emits laser light and receives laser light of a specific wavelength from the outside, the semiconductor laser including a semiconductor laser element that emits laser light, and a semiconductor in a laser light emitting direction. The length of the laser element is L, where L is the desired wavelength of the laser light emitted from the semiconductor laser element, λ is the allowable width of the wavelength of the laser light, and the refractive index of the semiconductor laser element Is a semiconductor laser determined by L ≧ (λ 2) / (2 × n × Δλ).

  According to the present invention, the length L of the semiconductor laser element used in the external resonator type semiconductor laser is suitably calculated by a predetermined formula, and a semiconductor laser element having the calculated length L or more is manufactured to externally By configuring the resonator type semiconductor laser, even if the output power or the operating temperature changes, the wavelength of the emitted laser light can be suppressed within a predetermined wavelength fluctuation range Δλ.

  In addition, according to the present invention, as described above, the wavelength of the emitted laser light can be suppressed to the range of the predetermined wavelength fluctuation range Δλ only by adjusting the length of the semiconductor laser element. There is no need to add a mechanism.

  The external cavity semiconductor laser according to the embodiment of the present invention is the same as the conventional configuration, and has the configuration shown in FIG. The laser light emitted from the semiconductor laser is made into a single mode by the external resonator type semiconductor laser having such a configuration, and for example, laser light having a wavelength as shown in FIG. 1 can be obtained. Here, FIG. 1 shows the output power of laser light for each wavelength (nm), and good single mode is realized in that the wavelength is about 409.609 nm. The vertical axis in FIG. 1 represents the relative output power of the emitted laser light.

  As described above, the center wavelength of a semiconductor laser that does not include an external resonator is linearly shifted to the longer wavelength side as the output power of the semiconductor laser increases (or as the operating temperature increases) ( That is, the wavelength increases in proportion to the output power). However, in the external resonator type semiconductor laser, the external resonator does not allow such a linear shift, and the wavelength of the emitted laser light periodically increases as shown in FIG. 2 as the output power increases. It turns out that it repeats descending.

  FIG. 2 shows the relationship between the output power and the wavelength of the emitted laser light for a semiconductor laser different from the semiconductor laser of FIG. 1, and the wavelength of the output power during the transition from 15 mW to 40 mW is shown. It shows a change. As can be seen from FIG. 2, when the output power rises to around 24 mW, the wavelength gradually changes to about 409.724 nm, about 409.734 nm, about 409.744 nm, and about 409.754 nm, and between 2 and 3 mW Respectively output laser light having a constant wavelength. When the output power passes around 24 mW, the wavelength drops rapidly to about 409.714 nm. Next, when the output power increases from around 24 mW to around 37 mW, the wavelength increases from about 409.714 nm to about 0.01 nm width and increases stepwise again to about 409.754 nm, and the output power reaches around 37 mW. After that, the wavelength drops rapidly and again becomes about 409.714 nm, and such a cycle is repeated.

  In the case of the example in FIG. 2, the fluctuation range of the wavelength of the laser light is about 0.040 nm (409.754 nm−409.714 nm = 0.040 nm), which is consistent with the mode hop interval of the laser chip. . Here, the principle that the wavelength of the laser beam emitted from the external cavity semiconductor laser changes periodically and stepwise will be described with reference to FIGS.

  3 to 6 show the modes of the external cavity semiconductor laser at a certain operating temperature, where the horizontal axis represents wavelength (nm) and the vertical axis represents relative output power (mW). Yes. The external cavity mode is a very fine longitudinal mode caused by the presence of light returning from the grating to the semiconductor laser, and the interval is inversely proportional to the distance between the semiconductor laser (laser diode) and the grating.

  On the other hand, the LD mode (internal mode) is possessed by the laser diode itself, and these shift to the longer wavelength side as the operating temperature increases. In the example of FIG. 3, the interval between the LD modes is 0.040 nm.

  The grating mode is light provided by the grating. The grating receives laser light of each mode from the laser diode and returns specific first-order diffracted light to the laser diode. The center frequency of the grating mode is almost constant regardless of the operating temperature.

  Of the many external resonator modes described above, the one that matches the center frequency of any LD mode and that has the output power of the grating mode corresponding to that wavelength has the wavelength closest to the output peak of the grating mode oscillates As a result, the wavelength of the laser light emitted from the external resonator type semiconductor laser becomes the wavelength.

  In the case shown in FIG. 3, there are three external resonator modes that have the same wavelength as the center frequency of the LD mode and are included in the wavelength of the grating mode, of which the closest to the output peak of the grating mode. The one is an external resonator mode of wavelength A, and the output power of the grating mode corresponding to this wavelength A is A1. It can be seen that the output powers of the grating modes corresponding to the wavelengths of the other two external resonator modes are A2 and A3, respectively, and are smaller than A1.

  The time when the state shown in FIG. 3 is reached corresponds to, for example, the time when the wavelength becomes about 409.734 nm in FIG. 2 (that is, A≈409.734 nm and the output power is about 29 mW).

  FIG. 4 shows a case where the center frequency of the LD mode in FIG. 3 is shifted from the wavelength A to the wavelength B which is the wavelength of another external resonator mode due to the increase of the operating temperature. Then, the output power of the grating mode at the wavelength B is B1, and the output powers of the grating modes corresponding to the wavelengths of the other external resonator modes are B2 and B3 smaller than B1. Therefore, B1 is closest to the output peak of the grating mode, and the wavelength of the laser light emitted from the external cavity semiconductor laser is shifted to B here.

  The time when the state shown in FIG. 4 is reached corresponds to the time when the wavelength becomes about 409.744 nm in FIG. 2 (that is, B≈409.744 nm and the output power is about 32 mW).

  FIG. 5 shows a case where the center frequency of the LD mode in FIG. 4 is shifted from the wavelength B to the wavelength C which is the wavelength of another external resonator mode due to the increase of the operating temperature. Then, the output power of the grating mode at the wavelength C is C1, and the output powers of the grating modes corresponding to the wavelengths of the other external resonator modes are C2 and C3 which are smaller than C1. Accordingly, C1 is closest to the output peak of the grating mode, and the wavelength of the laser light emitted from the external cavity semiconductor laser is shifted to C here.

  The time when the state shown in FIG. 5 is reached corresponds to the time when the wavelength becomes about 409.754 nm in FIG. 2 (that is, C≈409.754 nm and the output power is about 34 mW).

  Next, FIG. 6 shows a case where the center frequency of the LD mode in FIG. 5 is shifted from the wavelength C to the wavelength D, which is a wavelength of another external resonator mode, due to an increase in operating temperature. Then, the output power of the grating mode at the wavelength D is D2. However, the output power D1 of the grating mode corresponding to the wavelength of the other external resonator mode (the center frequency of the other LD mode) E is closer to the output peak of the grating mode than D2, and thus is emitted from the external resonator type semiconductor laser. The wavelength of the emitted laser light is shifted to E instead of D.

  The time when the state shown in FIG. 6 is reached corresponds to the time when the wavelength transitions from about 409.754 nm to about 409.714 nm in FIG. 2 (ie, E≈409.714 nm and the output power is about 37 mW). To do.

Thus, the wavelength of the laser light emitted from the external cavity semiconductor laser fluctuates stepwise and periodically in the range of about 409.714 nm to about 409.754 nm, and the LD mode interval (between the center frequencies). It does not change with the fluctuation range exceeding (distance). As described above, this interval is equal to the mode hop interval Δλ, and is expressed by the following Equation 1.
Δλ = (λ ^ 2) / (2nL) (Formula 1)
λ is the wavelength of the laser light emitted to the outside in the external cavity semiconductor laser, and n is the refractive index (effective refractive index) of the semiconductor laser element. L is the length of a semiconductor laser element (for example, a laser diode (laser chip)).

  Here, the length L of the laser chip will be described with reference to FIGS. FIG. 7 shows in detail the configuration of the semiconductor laser 1 of the present invention, which is the same as the semiconductor laser 55 of the conventional external cavity semiconductor laser 50 shown in FIG.

  The semiconductor laser 1 includes a window glass 2, a laser diode 3, a submount 4, a heat sink 5, an LD can 6, a stem 7, and terminal pins 8. The laser light 9 is emitted from the light emitting surface of the laser diode 3, passes through the window glass 2, and is emitted to the outside. The laser diode 3 is connected to a heat sink 5 for heat dissipation through a submount 4. The heat sink 5 is further connected to the stem 7 of the semiconductor laser 1.

  The laser diode 3, the submount 4, and the heat sink 5 are surrounded and sealed by the window glass 2, the LD can 6 and the stem 7. The terminal pin 8 is connected to the n electrode, the p electrode, and the like of the laser diode 3 and provides a predetermined current to the laser diode 3.

  FIG. 8 is a diagram showing the laser diode 3, the submount 4, the heat sink 5 and the like shown in FIG. 7 in more detail. The laser light 9 is emitted from the light emitting surface 14 of the laser diode 3. The laser diode 3 includes an active layer 11, a layer 12, and a layer 13.

  Here, the length L is the length of the laser diode 3 in the laser beam emission direction of the laser diode 3, as shown in FIG. 8, and according to another expression, the active layer 11 and other layers This is the length of the active layer 11 along an axis that is substantially parallel to at least one of the boundary surfaces 12 and 13 and substantially perpendicular to the light emitting surface 14.

  As described above, the wavelength λ is the wavelength of the laser light emitted from the external resonator type semiconductor laser, and the refractive index n is determined by the semiconductor material, and therefore cannot be freely changed. However, it is relatively easy to change L which is the length of the laser diode 3. According to Equation 1, the greater the length L, the smaller the wavelength variation due to output power variation. In addition, when L is increased, there is a secondary effect that the output power is increased, which is particularly advantageous when used for recording in a holographic memory.

  However, increasing L means that the number of chips that can be produced from one wafer is reduced, resulting in an increase in the production cost of each laser diode. Therefore, the magnitude of L should be carefully determined after fully considering such a trade-off relationship.

  It is also important to determine the allowable fluctuation range of the wavelength for each application field (application) of the laser light and take them into consideration.

  Assuming that the wavelength of the laser beam emitted to the outside is 405 nm and the refractive index is 3.6, L can be obtained as follows.

  In order to make the allowable width of the wavelength within 0.04 nm, L may be set to 570 μm or more, for example (L ≧ (λ ^ 2) / (2nΔλ) = (405 nm ^ 2) / (0.04 nm × 2 × 3.6) ≈569.5 μm).

  Further, in order to make the allowable width of the wavelength within 0.03 nm, L may be set to, for example, 760 μm or more (L ≧ (λ ^ 2) / (2nΔλ) = (405 nm ^ 2) / (0. 03 nm × 2 × 3.6) ≈759.4 μm).

  In order to make the allowable width of the wavelength within 0.02 nm, L may be set to 1140 μm or more, for example (L ≧ (λ ^ 2) / (2nΔλ) = (405 nm ^ 2) / (0.02 nm × 2 × 3.6) ≈1139.1 μm).

That is, the length L of the laser diode is obtained from the following formula 2, and the semiconductor laser element may be manufactured so as to have the obtained length L.
L ≧ (λ ^ 2) / (2nΔλ) (Expression 2)

  As described above, the external resonator type semiconductor laser according to the present invention is a semiconductor laser device (used for an external resonator type semiconductor laser) so that the wavelength variation of the emitted laser light falls within a predetermined range. For example, the length L of the laser diode) is preferably selected, whereby the wavelength of the laser light emitted from the external resonator type semiconductor laser is increased as the output power or the operating temperature increases. It varies periodically in the range of λ. Therefore, the present invention can be applied to various types of semiconductor laser elements including, for example, a blue laser diode, a green laser diode, and a red laser diode.

  The laser diode used in the Littrow type external cavity semiconductor laser has been described so far, but the laser diode according to the present invention may be used in the Littman type external cavity semiconductor laser. The same effect can be obtained.

It is a basic diagram showing the relationship between the wavelength of the laser beam radiate | emitted from a semiconductor laser, and output power. In an external resonator type semiconductor laser, it is a basic diagram showing the relationship between the wavelength of laser light emitted to the outside and the output power. It is a basic diagram showing the relationship between the wavelength of each mode of an external resonator type semiconductor laser and output power at a certain operating temperature. It is a basic diagram showing the relationship between the wavelength of each mode of the external cavity semiconductor laser and the output power at different operating temperatures. Furthermore, it is a basic diagram showing the relationship between the wavelength of each mode of the external cavity semiconductor laser and the output power at different operating temperatures. It is a basic diagram showing the relationship between the wavelength of each mode of the external cavity semiconductor laser and the output power at yet another operating temperature. It is a basic diagram showing the structure of the semiconductor laser concerning this invention. FIG. 8 is a schematic diagram showing in detail a part of the semiconductor laser shown in FIG. 7 such as a laser diode. It is a basic diagram showing the structure of a conventional external cavity semiconductor laser. It is a basic diagram showing the relationship between the wavelength of laser light emitted from a conventional external cavity semiconductor laser and the output power.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 2 ... Window glass, 3 ... Laser diode, 4 ... Submount, 5 ... Heat sink, 6 ... LD can, 7 ... Stem, 8 ..Terminal pins, 11 ... active layer, 12 ... layer, 13 ... layer, 14 ... light emitting surface

Claims (4)

A semiconductor laser that emits laser light;
A collimating lens that emits the laser light received from the semiconductor laser as substantially parallel light;
A grating that receives the laser light via the collimating lens,
The first-order diffracted light from the grating is incident on the semiconductor laser through the collimating lens, thereby realizing a single mode.
The semiconductor laser includes a semiconductor laser element that emits the laser light,
The length of the semiconductor laser element in the emission direction of the laser light is L,
L is a case where the desired wavelength of the laser light emitted from the semiconductor laser element is λ, the allowable width of the wavelength of the laser light is Δλ, and the refractive index of the semiconductor laser element is n And an external cavity semiconductor laser characterized by L ≧ (λ ^ 2) / (2 × n × Δλ). The external cavity semiconductor laser according to claim 1,
An external cavity semiconductor laser, wherein the semiconductor laser element is a laser diode. In a semiconductor laser that emits laser light and receives laser light of a specific wavelength from the outside,
The semiconductor laser includes a semiconductor laser element that emits the laser light,
The length of the semiconductor laser element in the emission direction of the laser light is L,
L is a case where the desired wavelength of the laser light emitted from the semiconductor laser element is λ, the allowable width of the wavelength of the laser light is Δλ, and the refractive index of the semiconductor laser element is n In addition, the semiconductor laser is determined by L ≧ (λ ^ 2) / (2 × n × Δλ). The semiconductor laser according to claim 3, wherein
A semiconductor laser, wherein the semiconductor laser element is a laser diode.

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