Semiconductor laser device

Abstract

To provide an internal PD monitor-type semiconductor laser device in which long-term output instability due to the wavelength dependence of sensitivity of a PD can be improved.SOLUTION: A laser diode (LD) chip 110 is an end-face emission type and emits forward optical output L1 from a forward end face S1. A glass window 150 is provided on the forward end-face S1 side of the LD chip 110 and reflects a portion of the forward optical output L1. A light-receiving element 170 is provided on the rearward end-face S2 side of the LD chip 110 so that the light-receiving element can receive reflected forward optical output L4 that is forward optical output L3 reflected by the glass window 150. In an oscillation wavelength band in actual use of the semiconductor laser chip, the wavelength dependence of the sensitivity S of the light-receiving element is reverse to that of the reflectance Rwin of the glass window.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a semiconductor laser device. [Background technology]

[0002] There are semiconductor laser devices in which an edge-emitting semiconductor laser (LD: Laser Diode) chip is housed in a package such as a CAN package together with a photodiode (PD: Photo Diode). The internal PD built into the package is placed on the rear end face side of the LD chip and receives the rear optical output leaking from the rear end face of the LD chip. Since the power of the rear optical output correlates with the power of the light (forward optical output) emitted from the front end face of the LD chip, the output of the internal PD indirectly indicates the power of the forward optical output of the LD chip.

[0003] The output of the internal PD is taken out to the outside of the package. The driving circuit outside the package feedback controls the driving current supplied to the LD chip so that the output of the internal PD is constant (APC: Automatic Power Control). This makes it possible to stabilize the forward optical output of the LD chip.

[0004] The internal PD monitor method indirectly monitors the forward optical output Pf by using the rear optical output Pr. In this method, the ratio Pr / Pf of Pr to Pf is required to be constant regardless of temperature or wavelength. The reflectance of the front end face of the LD chip is Rf, and the reflectance of the rear end face is Rr. The power of the light emitted from the front end face Pf and the power of the light emitted from the rear end face Pr each satisfy the following relationship. Pf ∝ 1 / √Rf × (1-Rf) Pr ∝ 1 / √Rr × (1-Rr)

[0005] Therefore, the ratio of Pr to Pf, Pr / Pf, is expressed by equation (1). Pr / Pf=√Rf / √Rr×(1-Rr) / (1-Rf) …(1)

[0006] In most cases, inexpensive Si is used for the internal PD, and its sensitivity is wavelength dependent, with the sensitivity being particularly low on the short wavelength side of the oscillation wavelength band of GaN / InGaN LD chips. On the other hand, the oscillation wavelength of GaN / InGaN LDs shifts to the long wavelength side as the external environmental temperature increases. Therefore, when controlling the drive current of the LD chip to keep the output of the internal PD constant, there is a problem in that the fluctuation range of the decrease / increase in the laser output associated with the increase / decrease in temperature is large. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 5319397 Summary of the Invention [Problem to be solved by the invention]

[0008] Patent Document 1 discloses a technique for designing the facet reflectivities Rr and Rf of an LD chip so that Pr / Pf expressed by equation (1) has wavelength dependency in the opposite direction to the wavelength dependency of the PD sensitivity.

[0009] This technology requires different adjustments for each product with different values ​​for at least one of the LD chip facet reflectances Rr and Rf, and requires verification that the monitor PD sensitivity and output relative values ​​fall within the allowable range as designed. Therefore, there is a problem that it is a heavy burden in horizontal expansion to products with different reflective film structures or in development to apply new reflective film structures.

[0010] The present disclosure has been made in consideration of these problems, and one exemplary purpose of an embodiment of the present disclosure is to provide a semiconductor laser device with an internal PD monitor type that improves output instability caused by the wavelength dependency of PD through a structure different from that of a conventional device. [Means for solving the problem]

[0011] An embodiment of the present disclosure relates to a semiconductor laser device. The semiconductor laser device includes an edge-emitting semiconductor laser chip that emits forward optical output from a front end face, a glass window that is provided on the front end face side of the semiconductor laser chip and reflects a part of the forward optical output, and a light-receiving element that is provided on the rear end face side of the semiconductor laser chip so as to receive the reflected forward optical output that is the forward optical output reflected by the glass window. In the practical oscillation wavelength band of the semiconductor laser chip, the sensitivity S of the light-receiving element and the reflectance Rwin of the glass window have inverse wavelength dependence.

[0012] In addition, any combination of the above components, and mutual substitution of the components and expressions of the present disclosure between methods, devices, systems, etc. are also valid aspects of the present disclosure. Effect of the Invention

[0013] According to an embodiment of the present disclosure, the instability of the output caused by the wavelength dependency of the PD can be improved by using a structure different from the conventional one. [Brief description of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view of a semiconductor laser device according to a first embodiment. [Diagram 2] 2 is a diagram illustrating the operation of the semiconductor laser device of FIG. [Diagram 3] FIG. 11 is a cross-sectional view of a semiconductor laser device according to a second embodiment. [Figure 4] FIG. 4 is a plan view of the semiconductor laser device of FIG. [Diagram 5] FIG. 11 is a cross-sectional view of a semiconductor laser device according to a third embodiment. [Figure 6] FIG. 1 is a cross-sectional view of a semiconductor laser device simplified for design purposes. [Figure 7] FIG. 13 is a diagram showing the powers Pf”, Pr′ of light incident on a light receiving element when Rwin=5%. [Figure 8] FIG. 13 is a diagram showing the powers Pf”, Pr′ of light incident on a light receiving element when Rwin=3%. [Figure 9]FIG. 13 is a diagram showing the powers Pf”, Pr′ of light incident on a light receiving element when Rwin=2%. [Figure 10] FIG. 13 is a diagram showing the powers Pf”, Pr′ of light incident on a light receiving element when Rwin=1%. [Figure 11] FIG. [Figure 12] FIG. 2 is a cross-sectional view of an LD chip according to an embodiment. [Figure 13] 13 is a diagram showing the reflectance and transmittance of the rear end face of the LD chip in FIG. 12. [Figure 14] FIG. 2 is a cross-sectional view of an LD chip according to an embodiment. [Figure 15] 15 is a diagram showing the transmittance of the rear end face of the LD chip in FIG. 14. [Figure 16] FIG. 2 is a cross-sectional view of an LD chip according to an embodiment. [Figure 17] FIG. 17 is a diagram showing the transmittance of the rear end face of the LD chip in FIG. 16. [Figure 18] FIG. 1 is a diagram showing the wavelength dependence of sensitivity S of a Si photodiode. [Figure 19] 4 is a cross-sectional view of a reflective film of a glass window according to Design Example 1. FIG. [Figure 20] FIG. 20 is a diagram showing the wavelength dependence of the optical characteristics of the reflective film of FIG. 19. [Figure 21] FIG. 11 is a cross-sectional view of a reflective film of a glass window according to Design Example 2. [Figure 22] FIG. 22 is a diagram showing the wavelength dependence of the optical characteristics of the reflective film of FIG. 21. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] (Overview of the embodiment) A summary of some exemplary embodiments of the present disclosure is provided. This summary is intended as a prelude to the more detailed description that follows, or as a basic understanding of the embodiments. This summary is intended to provide a simplified description of some concepts of one or more embodiments, and is not intended to limit the scope of the invention or disclosure. Additionally, this summary is not intended to be a comprehensive overview of all possible embodiments, nor is it intended to limit essential elements of the embodiments. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed in this specification.

[0016] A semiconductor laser device according to an embodiment includes an edge-emitting semiconductor laser chip that emits a forward optical output from a front end face, a glass window that is provided on the front end face side of the semiconductor laser chip and reflects a part of the forward optical output, and a light-receiving element that is provided on the rear end face side of the semiconductor laser chip so as to receive the reflected forward optical output that is the forward optical output reflected by the glass window. In the practical oscillation wavelength band of the semiconductor laser chip, the sensitivity S of the light-receiving element and the reflectance R of the glass window have inverse wavelength dependence.

[0017] According to this configuration, the influence of the wavelength dependency of the light receiving element can be cancelled by utilizing the wavelength dependency of the reflectance and transmittance of the glass window. In the conventional technology that cancels the wavelength dependency of the light receiving element by adjusting the end face reflectances Rr and Rf of the LD chip, it was necessary to optimize the reflectances Rr and Rf for each LD chip product. In contrast, in one embodiment, the reflectance and transmittance of the glass window can be designed independently of the end face reflectance of the LD chip. Therefore, it is possible to reduce the burden in horizontal expansion to LD chips with different reflective film structures and development of applying new reflective film structures. In addition, the reflectance and transmittance of the glass window are more stable than the reflective film of the end face of the LD chip, so that a stable light output can be obtained for a long time.

[0018] In one embodiment, in the actual oscillation wavelength band of the semiconductor laser chip, when the sensitivity of the light receiving element is S(λ) and the reflectance of the glass window is Rwin(λ), the reflection / transmission intensity ratio β(λ)=Rwin(λ) / (1-Rwin(λ)) may have a wavelength dependency that is inverse to that of the sensitivity S(λ).

[0019] In one embodiment, a wavelength in the practical oscillation wavelength band is denoted by λ0, and normalized sensitivity is denoted by S norm The normalized sensitivity S(λ)=S(λ) / S(λ0) norm When (λ) has a slope α, the normalized reflected / transmitted intensity ratio β norm (λ)=β(λ) / β(λ0) may be between the two straight lines {1-α(λ-λ0)}×0.95 and {1-α(λ-λ0)}×1.05 in the practical oscillation wavelength band. This makes it possible to suppress the fluctuation of the optical output to within ±5%.

[0020] In one embodiment, Rwin may be ≧3%. This makes it possible to suppress the fluctuation width of the optical output to 5% or less by fine-tuning the reflective film of two sets of five-layer bandpass or four sets of five-layer bandpass formed on the glass window for a light receiving element with a slope rate of the wavelength dependency of sensitivity of up to 3% / nm.

[0021] In one embodiment, the power of the rear optical output emitted from the rear end facet of the semiconductor laser chip that is incident on the light receiving element may be ⅓ or less of the power of the reflected forward optical output that is incident on the light receiving element.

[0022] The light incident on the light receiving element may include the rear optical output emitted from the rear end face of the semiconductor laser chip and the reflected forward optical output, but the power of the reflected forward optical output is more than twice as much as the power of the rear optical output, so the optical output from the front is dominant. In the conventional internal PD method, the forward optical output is indirectly monitored using the rear optical output, so the ratio of the power of the rear optical output and the power of the forward optical output of the semiconductor laser chip is required to be constant. In contrast, the power of the forward optical output is directly monitored, so the ratio of the power of the rear optical output and the power of the forward optical output of the semiconductor laser chip is less susceptible to long-term fluctuations. In addition, the power of the reflected forward optical output is determined by the reflectance of the glass window, but the reflectance of the glass window is more stable in the long term than the reflective film formed on the end face of the semiconductor laser chip. Therefore, according to the above configuration, the long-term stability of the output can be improved compared to the conventional internal PD method.

[0023] In one embodiment, the power of the rear optical output incident on the light receiving element may be 1 / 10 or less of the power of the reflected forward optical output incident on the light receiving element, thereby further reducing the effect of long-term fluctuations in the ratio of the power of the rear optical output to the power of the forward optical output of the semiconductor laser chip, thereby improving long-term stability.

[0024] In one embodiment, the reflectance Rwin of the glass window may be 2% or greater. Rwin≧2% This makes it easier to meet the design condition that the power of the backward optical output incident on the photodetector is 1 / 10 or less of the power of the reflected forward optical output incident on the photodetector for combinations of typically expected package sizes and semiconductor laser chips with various specifications.

[0025] In one embodiment, a reflective coating may be formed on both sides of the glass window. In one embodiment, a reflective coating may be formed on an inner surface of the glass window. In one embodiment, a reflective coating may be formed on an outer surface of the glass window.

[0026] In one embodiment, the reflectance of the rear reflection film formed on the rear end face of the semiconductor laser chip may be 99.5% or more, which makes it easier to satisfy the design condition that the power of the rear optical output incident on the light receiving element is 1 / 10 or less of the power of the reflected forward optical output incident on the light receiving element for combinations of normally assumed package sizes and semiconductor laser chips with various specifications.

[0027] In one embodiment, the reflectance of the rear reflection film formed on the rear end face of the semiconductor laser chip may be 99.9% or more, which makes it easier to satisfy the above design conditions.

[0028] In one embodiment, when the semiconductor laser device is viewed along the emission direction of the forward optical output, the light receiving element may be offset from the semiconductor laser chip, thereby reducing the power of the rear optical output incident on the light receiving element.

[0029] In one embodiment, the semiconductor laser chip may include an absorption layer formed on a rear reflection film formed on the rear facet, which can reduce the power of the rear optical output incident on the photodetector.

[0030] In one embodiment, the absorbing layer may include a high refractive index film or may include a metal film with a large extinction coefficient.

[0031] In one embodiment, the transmittance of the absorbing layer may be 1% or less.

[0032] (Embodiment) Hereinafter, the present disclosure will be described with reference to the drawings based on preferred embodiments. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and duplicated descriptions will be omitted as appropriate. In addition, the embodiments are not intended to limit the disclosure, but are merely examples, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure.

[0033] The dimensions (thickness, length, width, etc.) of each component shown in the drawings may be enlarged or reduced as appropriate for ease of understanding. Furthermore, the dimensions of multiple components do not necessarily represent their relative sizes. Even if a component A is drawn thicker than another component B on the drawings, component A may actually be thinner than component B.

[0034] (Embodiment 1) 1 is a cross-sectional view of a semiconductor laser device 100A according to embodiment 1. The semiconductor laser device 100A is a laser in a CAN package, and includes an LD chip 110, a submount 120, a metal stem 130, a metal cap 140, a glass window 150, leads 160, and a light receiving element 170.

[0035] The LD chip 110 is an edge-emitting laser, and emits a forward optical output from a front end face S1. The LD chip 110 is mounted on a submount 120, which is fixed to a metal stem 130. The oscillation wavelength of the LD chip 110 is not particularly limited, but the technology according to the present disclosure is useful in the oscillation wavelength band of GaN / InGaN. A front reflection film 112 having a relatively low reflectance is formed on the front end face S1 side of the LD chip 110, and a rear reflection film 114 having a relatively high reflectance is formed on the rear end face S2 side.

[0036] The metal cap 140 covers the LD chip 110 from the side, and supports and fixes the glass window 150. A reflective film 152 that reflects a part of the forward light output L1 is formed on the glass window 150. The reflective film 152 may be formed on both sides of the glass window 150, or may be formed on only one side. The reflectance of the glass window 150 on which the reflective film 152 is formed is Rwin. Here, the transmittance of the glass window 150 can be expressed as (1-Rwin).

[0037] A light receiving element (internal PD) 170 is provided on the rear end face S2 side of the LD chip 110 so as to receive the reflected forward light output, which is the forward light output L1 reflected by the reflective film 152. The light receiving element 170 is, for example, a Si photodiode. A typical metal stem 130 has an inclined surface (counterbore surface) 132 formed so that a light receiving surface 172 of the light receiving element 170 is non-parallel to the rear end face S2 of the LD chip 110. This is to prevent the rear light output emitted from the rear end face S2 of the LD chip 110 from being reflected by the light receiving element 170 and returning to the LD chip 110.

[0038] The multiple leads 160 penetrate the metal stem 130 in a manner insulated from the metal stem 130. The LD chip 110 and the light receiving element 170 are electrically connected to the corresponding leads 160. The multiple leads 160 are electrically connected to a drive circuit (not shown).

[0039] The above is the configuration of the semiconductor laser device 100A. Next, the operation of the device will be described.

[0040] Fig. 2 is a diagram for explaining the operation of the semiconductor laser device 100A of Fig. 1. Light beams L1 to L6 are shown in Fig. 2. In Fig. 2, some members that are not related to the explanation of the light beams L1 to L6 are omitted.

[0041] A forward optical output L1 is emitted from the front end surface S1 of the LD chip 110. The power of the forward optical output L1 is denoted as Pf. Most of the forward optical output L1 passes through the glass window 150 and is extracted to the outside as output light L2 of the semiconductor laser device 100A. The power Po of the output light L2 is expressed by equation (2). Po = Pf × (1 - Rwin) … (2) (1-Rwin) represents the transmittance of the glass window 150.

[0042] A portion of the forward light output L1 is reflected by the reflective film 152 of the glass window 150. The portion of the forward light output L1 reflected by the reflective film 152 is referred to as a reflected forward light output L3, and its power is referred to as Pf'. Pf' = Pf × Rwin … (3)

[0043] A portion L4 of the reflected forward light output L3 is incident on the light receiving surface 172 of the light receiving element 170. The power of the incident light L4 on the light receiving surface 172 is designated as Pf". In the following description, Pf" / Pf' is referred to as the light receiving rate ηf. The light receiving rate ηf is the proportion of the light L4 that is incident on the light receiving surface 172 of the light receiving element 170 out of the reflected forward light output L3. Pf” = Pf’ × ηf … (4)

[0044] A part L6 of the rear optical output L5 emitted from the rear end face S2 of the LD chip 110 is incident on the light receiving surface 172 of the light receiving element 170. When the power of the rear optical output L5 is Pr and the power of the incident light L6 is Pr', Pr' / Pr is the light receiving rate ηr. Pr' = Pr × ηr … (5)

[0045] An incident light L4 from the front and an incident light L6 from the rear are incident on a light receiving surface 172 of the light receiving element 170. The light receiving element 170 generates an electrical signal according to the total power (called detection power) Pdet of the two incident lights L4 and L6. Pdet = Pf” + Pr’ … (6)

[0046] In a conventional semiconductor laser device with an internal PD, the light L6 coming from the rear is a signal component among the light incident on the light receiving element 170, and the rest are noise components. In contrast, in this embodiment, the light incident on the light receiving element 170 is dominated by the incident light L4 coming from the front, which is the main signal component. In other words, the semiconductor laser device 100A is configured to be able to monitor the output light L2 mainly based on the light L4 coming from the front.

[0047] "The incident light L4 is dominant" means that the power Pf" of the reflected forward light output L4 is three times or more the power Pr' of the rearward light output L6. In other words, the power Pr' of the rearward light output L6 incident on the light receiving element 170 is 1 / 3 or less of the power Pf" of the reflected forward light output L4 incident on the light receiving element 170.

[0048] The above is the operation of the semiconductor laser device 100A. Next, its advantages will be described.

[0049] In this semiconductor laser device 100A, a forward optical output L1 is reflected by a reflective film 152 formed on a glass window 150, and an incident light L4 based on the reflected forward optical output L3 is detected by a light receiving element 170.

[0050] By substituting equations (1) to (5) into equation (6), we obtain equation (7). Pdet=Pf”+Pr'=Pf×Rwin×ηf+Pr×ηr …(7)

[0051] Assuming that the power Pf'' of the reflected forward light output L4 is sufficiently larger than the power Pr' of the backward light output L6 (Pf'>>Pr') on the light receiving surface 172 of the light receiving element 170, equation (8) is obtained. Pdet ≒ Pf × Rwin × ηf … (8)

[0052] For example, the power Pr′ of the incident light L6 incident on the light receiving surface 172 of the light receiving element 170 is preferably 1 / 10 (ie, 0.1) or less of the power Pf″ of the reflected forward light output L4 incident on the light receiving surface 172. Pr' / Pf”≦0.1 …(9) By doing so, the detected power Pdet becomes less dependent on the light L6 coming from the rear, and is less susceptible to the influence of fluctuations and variations in the reflectance Rr of the rear end face S2 of the LD chip 110.

[0053] As can be seen from formula (8), the detection power Pdet is proportional to the reflectance Rwin and the light receiving rate ηf, but the light receiving rate ηf is determined by geometrical optics and can be regarded as a constant. In addition, since the reflecting film 152 is formed on the glass window 150 whose physical and optical properties are stable, its optical properties are very stable compared to the front reflecting film 112 on the front end face S1 and the rear reflecting film 114 on the rear end face S2 formed on the semiconductor, and therefore the reflectance Rwin is also stable compared to the reflectances Rf and Rr of the LD chip 110. Therefore, in this embodiment, by using the reflected forward optical output L3, it is possible to monitor the power Pf of the forward optical output L1 with high accuracy.

[0054] When feedback control is performed by an external driving circuit of the semiconductor laser device 100 so that the detection power Pdet is constant, the power Pf of the forward optical output L1 is kept constant, and thus the power Po of the external optical output L2 expressed by equation (2) is kept constant.

[0055] According to this embodiment, even if fluctuations in the LD characteristics (changes in Pr / Pf due to facet degradation) occur, including fluctuations in the facet reflectivity Rf or Rf of the LD chip during long-term power-on operation, the ratio of the external optical output L2 of the package to the internal PD monitor value is stably maintained, enabling accurate APC operation.

[0056] (Embodiment 2) Fig. 3 is a cross-sectional view of a semiconductor laser device 100B according to embodiment 2. Fig. 4 is a plan view of the semiconductor laser device 100B of Fig. 3. When the semiconductor laser device 100B is viewed from the emission direction of the forward optical output L1 of the LD chip 110, the light receiving element 170 is disposed offset in the Z direction with respect to the LD chip 110. In Fig. 4, a dashed line L4 indicates a range irradiated with the backward optical output L4.

[0057] According to the second embodiment, the light receiving rate ηr for the backward light output L5 is low, so that the incident light L6 incident on the light receiving surface 172 can be reduced. Conversely, the light blocked by the LD chip 110 for the reflected forward light output L3 is reduced, so that the light receiving rate ηf is high, so that the incident light L4 can be increased. This allows Pr' / Pf'' to be reduced.

[0058] (Embodiment 3) 5 is a cross-sectional view of a semiconductor laser device 100C according to the third embodiment. In the semiconductor laser device 100C, an absorption layer 116 is formed on a rear reflection film 114 at the rear end face S2 of the LD chip 110. The transmittance of the absorption layer 116 is set to 10% or less, preferably 5% or less, and more preferably 1% or less. The absorption layer 116 may be a high refractive index layer such as amorphous Si, or a metal film with a large extinction coefficient such as Au (gold).

[0059] By providing the absorption layer 116, the power of the rear optical output L6 incident on the light receiving surface 172 of the light receiving element 170 can be significantly reduced. This makes it possible to reduce the proportion of Pr' in the detection power Pdet and increase the proportion of Pf".

[0060] (Design example) FIG. 6 is a cross-sectional view of a semiconductor laser device 100 that has been simplified for design purposes. The dimensions are as follows: Lc: Resonator length of the LD chip 110 Lf: Distance from the front end surface S1 of the LD chip 110 to the reflecting surface of the glass window 150 Lr is the distance from the rear end surface S2 of the LD chip 110 to the inclined surface 132 within the plane 134 that is in contact with the bottom surface of the submount 120. α: inclination angle of inclined surface 132 D: Height from the plane 134 to the waveguide (active layer) of the LD chip 110 relative to the bottom surface of the submount 120 d Height from the inclined surface 132 to the light receiving surface 172 The distance from the center of the light-receiving surface 172 of the light-receiving element 170 along the inclined surface 132, which is the separation distance (shift amount) from the axis passing through the center of the waveguide of the LD chip 110 along the waveguide H: The vertical width of the light-receiving surface 172 of the light-receiving element 170 L: The horizontal width of the light-receiving surface 172 of the light-receiving element 170

[0061] The following were used for each dimension. D = 0.3 mm d = 0.3 mm α = 13° Lf = 0.5 mm Lr = 0.3 mm, 0.5 mm, 0.7 mm Lc = 0.6 mm, 0.8 mm, 1.0 mm, 1.2 mm, 1.5 mm, 2.0 mm H = 0.5 mm L = 0.5 mm

[0062] For the forward light output L1 and the backward light output L5 emitted from the LD chip 110, assuming a Gaussian profile, the divergence angle θv of the beam in the vertical direction (Z direction) was set to 22°, and the divergence angle θh of the beam in the horizontal direction (Y direction) was set to 9°.

[0063] Figure 7 is a diagram showing the powers Pf” and Pr’ of the light incident on the light-receiving element 170 when Rwin = 5%. The four graphs correspond to Rr = 90%, 99%, 99.5%, and 99.7%. Rf = 10% is common. The vertical axis shows the relative power normalized with the forward light output L1 as 1 on a logarithmic axis. The horizontal axis shows the position of the light-receiving element 170 (Ls in Figure 6).

[0064] Each figure also shows Pf” × 0.33 and Pf” × 0.1 together. ·When Rr = 90%, in the range of Ls ≥ 0.34 mm, the design condition Pr’ < Pf” × 0.1 is satisfied, and in the range of Ls ≥ 0.32 mm, Pr’ < Pf” × 0.33 is satisfied. · When Rr = 99%, in the range of Ls ≥ 0.26 mm, the design condition Pr’ < Pf” × 0.1 is satisfied, and in the range of Ls ≥ 0 mm, Pr’ < Pf” × 0.33 is satisfied. That is, in the design condition Pr’ < Pf” × 0.33, the shift of the light receiving element 170 is unnecessary. · When Rr = 99.5%, in the range of Ls ≥ 0.20 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99.7%, in the range of Ls ≥ 0 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. That is, the shift of the light receiving element 170 is unnecessary.

[0065] Assume that the sensitivity of the PD typically used as the light receiving element 170 is 0.16 A / W. When the power Po of the external light output L2 is 60 mW during operation, in order to ensure an output current of 50 μA for the light receiving element 170, Pf” > 0.005 is required. Therefore, from this constraint condition, the upper limit of the shift amount Ls of the light receiving element 170 is 0.68 mm.

[0066] Figure 8 is a diagram showing the powers Pf”, Pr’ of the light incident on the light receiving element 170 when Rwin = 3%. The four graphs correspond to Rr = 90%, 99%, 99.5%, 99.9%. Rf = 10% is common. · When Rr = 90%, in the range of Ls > 0.36 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99%, in the range of Ls ≥ 0.28 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99.5%, in the range of Ls ≥ 0.26 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99.9%, in the range of Ls ≥ 0 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. That is, the shift of the light receiving element 170 is unnecessary.

[0067] Also, the upper limit of the shift amount Ls of the light receiving element 170 that satisfies the condition Pf” > 0.005 to ensure the output current of the PD is 0.54 mm.

[0068] Also, when reducing the reflectance Rwin of the glass window 150, the reflectance Rf of the front end face S1 that can make the shift amount Ls of the light receiving element 170 zero increases.

[0069] FIG. 9 is a diagram showing the powers Pf” and Pr’ of the light incident on the light receiving element 170 when Rwin = 2%. The four graphs correspond to Rr = 90%, 99%, 99.5%, and 99.9%. Rf = 10% is common.

[0070] · When Rr = 90%, in the range of Ls > 0.38 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99%, in the range of Ls ≥ 0.30 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99.5%, in the range of Ls ≥ 0.28 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. · When Rr = 99.9%, in the range of Ls ≥ 0 mm, the design condition Pr’ < Pf” × 0.1 is satisfied. That is, the shift of the light receiving element 170 is unnecessary.

[0071] Also, the upper limit of the shift amount Ls of the light receiving element 170 that satisfies the condition Pf” > 0.005 to ensure the output current of the PD is 0.40 mm.

[0072] FIG. 10 is a diagram showing the powers Pf” and Pr’ of the light incident on the light receiving element 170 when Rwin = 1%. The four graphs correspond to Rr = 90%, 99%, 99.5%, and 99.9%. Rf = 10% is common. When reducing Rwin to 1%, the condition Pf” > 0.005 to ensure the output current of the PD cannot be satisfied.

[0073] The following findings can be obtained from the comparison of FIGS. 7 to 10. · When reducing the reflectance Rwin of the glass window 150, the shift amount Ls of the light receiving element 170 required to satisfy the design conditions increases. · When reducing the reflectance Rwin of the glass window 150, the upper limit of the shift amount of the light receiving element 170 also decreases. The lower the reflectance Rwin, the narrower the range in which the light receiving element 170 can be arranged. In other words, the higher the reflectance Rwin, the wider the range in which the light receiving element 170 can be arranged.

[0074] Considering the normally possible end face reflectivities Rf, Rr of the LD chip 110 and the shift amount Ls of the light receiving element 170, it is preferable to set Rwin ≧ 2%, and if Rwin ≧ 3%, the degree of freedom in designing other parameters increases. Also, if Rwin is too high, the forward output will be low, so Rwin ≦ 30% is preferable.

[0075] Furthermore, a realistic distance Lr between the LD chip 110 and the inclined surface 132 is about 0.5 mm. Assuming that Lr ≈ 0.5 mm, if a design that satisfies Rwin ≧ 3% and Rr ≧ 99.93% is adopted, the above-mentioned design conditions can be generally satisfied for various specifications of the LD chip 110.

[0076] (Design of glass window 150) Next, the reflectance Rwin of the glass window 150 will be described.

[0077] 11 is a cross-sectional view of glass window 150. The reflectance of the inner surface of glass window 150 is Ri, and the reflectance of the outer surface is Ro. The effective reflectance Rwin of glass window 150 is expressed by equation (10). Rwin=(Ri+Ro-2Ro·Ri) / (1-Ri·Ro) …(10)

[0078] For example, when Ro<<Ri, equation (10) can be approximated by equation (11). Rwin ≒ Ri + Ro (1 - Ri) 2 …(11) Furthermore, when Ro is small (for example, Ro<0.3%), it can be approximated by equation (12). Rwin≒Ri …(12)

[0079] Therefore, it may be designed so that Ri≧3%, Ro<0.3%. In this case, a reflective film 152 may be formed on the inner surface of the glass window 150, and an anti-reflective film (AR coat) 154 may be formed on the outer surface of the glass window 150.

[0080] If the material of the glass window 150 is SiO2, the anti-reflection film 154 may be designed to have, for example, a four-layer structure as shown below. air 4th layer SiO242nm 3rd layer TiO267nm 2nd layer SiO254nm 1st layer TiO274nm Glass window SiO2

[0081] This antireflection film 154 can achieve a reflectance of 0.1% or less in the practical wavelength band of 395 nm to 415 nm. Therefore, the reflectance Rwin can be designed by taking into account only the inner reflective film 152.

[0082] The reflective film 152 and the anti-reflective film 154 may be switched from the inside to the outside to satisfy the conditions Ri<<Ro, Ro≧3%, Ri<0.3%.

[0083] When Ri=Ro=R, the reflectance Rwin is expressed by equation (13). Rwin = 2R / (1+R) …(13) If we reverse-calculate equation (13) with respect to R, we get equation (14). R = Rwin / (2 - Rwin) ... (14) Therefore, when the design value of Rwin is Rd, the reflectance 152 may be designed to satisfy the relational expression (15). Ro = Ri = Rd / (2 - Rd) ... (15) In addition, in equation (14), if Rwin is 10% or less, it can be assumed that (2-Rwin) ≒ 2 with an accuracy of 5%, and equation (13) can be approximated by the following equation (13'). Rwin≒2R…(13') Therefore, when the design value of Rwin is set to Rd, the reflective film 152 may be designed so as to satisfy Ro = Ri = Rd / 2.

[0084] Also, when Ro ≠ Ri and Ro is determined in advance to be a fixed value, by inverse calculation of Equation (10) with respect to Ri, it is expressed by Equation (16). Ri = (Rwin - Ro) / (1 - 2·Ro + Rwin·Ro) …(16) Therefore, when Ro is an arbitrary fixed value and the design value of Rwin is set to Rd, the reflectivity film 152 may be designed so as to satisfy the relational expression (17). Ri = (Rd - Ro) / (1 - 2·Ro + Rd·Ro) …(17) In addition, when Ri is determined in advance to be a fixed value instead of Ro, by inverse calculation of Equation (10) with respect to Ro, it is expressed by Equation (18). Ro = (Rwin - Ri) / (1 - 2·Ri + Rwin·Ri) …(18) Therefore, when Ri is an arbitrary fixed value and the design value of Rwin is set to Rd, the reflective film 154 may be designed so as to satisfy the relational expression (19). Ro = (Rd - Ri) / (1 - 2·Ri + Rd·Ri) …(19) Also, when Ro or Ri is fixed in advance, for example, when Ro is fixed and Ro ≤ 5%, Rwin < 2Ro / (1 + Ro) ≤ 9.5%, since Ri < Ro ≤ 5% is satisfied, Equation (10) can be approximated by Equation (10') within an accuracy of 5%. Rwin ≒ Ri + Ro …(10’) Therefore, when the design value of Rwin is set to Rd smaller than 9.5%, the reflective film 152 and the reflective film 154 may be designed so as to satisfy Ri ≤ 5%, Ro ≤ 5%, and Rd = Ri + Ro.

[0085] (Design of Reflective Film of LD Chip 110) FIG. 12 is a cross-sectional view of an LD chip 110 according to an embodiment. As described above, a high reflectivity Rr is required for the rear reflective film 114 of the LD chip 110. The rear reflective film 114 can be formed of a dielectric multilayer film.

[0086] The rear reflection film 114 may include an underlayer 114a made of AlN and a dielectric multilayer film 114b. Assuming that the oscillation wavelength band of the LD chip 110 is 395 nm to 415 nm, the dielectric multilayer film 114b may have a multilayer structure of a SiO2 layer and a TiO2 layer. When λ=405 nm and the refractive index n of SiO2 is 1.49, the thickness of the SiO2 layer is 67 nm. When the refractive index n of TiO2 is 2.82, the thickness of the TiO2 layer is 36 nm. The dielectric multilayer film 114b is terminated by a SiO2 layer having a thickness of λ / 2n (=134 nm).

[0087] Fig. 13 is a diagram showing the reflectance Rr and transmittance of the rear end facet of the LD chip 110 in Fig. 12. N is the number of pairs of SiO2 layers and TiO2 layers. The thickness of the underlying AlN layer is 20 nm.

[0088] FIG. 14 is a cross-sectional view of an LD chip 110 according to an embodiment. In this LD chip 110, an absorption layer 116 is formed on a dielectric multilayer film 114b consisting of N layers. The absorption layer 116 includes a SiO2 layer 116a having a thickness of λ / 4n, a high refractive index layer 116b, and a termination layer 116c. The high refractive index layer 116b has a thickness of 3λ / 4. The high refractive index layer 116b can be made of a material having a refractive index of 4 or more, and a-Si (amorphous silicon) having a refractive index of 5.42 is suitable. The transmittance of the absorption layer 116 is preferably 1% or less, which allows the reflectance of the dielectric multilayer film 114b to be designed to be low, in other words, the number of pairs N to be small.

[0089] Fig. 15 is a diagram showing the transmittance of the rear end face of the LD chip 110 of Fig. 14. For comparison, the left side of Fig. 15 shows the transmittance when the high refractive index layer 116b is not included. The addition of the high refractive index layer 116b has the same effect as increasing the number of pairs of dielectric multilayer films by two.

[0090] 16 is a cross-sectional view of an LD chip 110 according to one embodiment. The absorption layer 116 of this LD chip 110 includes a metal layer 116d. For the metal layer 116d, a material with a large extinction coefficient k should be selected, for example, Au with k=1.96.

[0091] Fig. 17 is a diagram showing the transmittance of the rear end facet of the LD chip 110 of Fig. 16. For comparison, the transmittance without the metal layer 116d is shown on the left of Fig. 17. The addition of the metal layer 116d has the same effect as increasing the number of pairs of dielectric multilayer films by three.

[0092] (About wavelength dependency) Next, a design for further stabilizing the output of the semiconductor laser device 100 will be described. In the oscillation wavelength band of the LD chip 110, the sensitivity of the light receiving element 170 may have wavelength dependency. For example, when a Si photodiode is used as the light receiving element 170 and a GaN / InGaN-based material is used as the LD chip 110, the sensitivity of the Si photodiode has a positive wavelength dependency in the oscillation wavelength band of the LD chip 110. Since the oscillation wavelength of the LD chip 110 shifts due to temperature fluctuations, the sensitivity of the Si photodiode changes depending on the temperature. In an application in which the semiconductor laser device 100 is used in an environment without temperature management, if feedback control is performed to keep the output of the photodiode constant, the oscillation wavelength fluctuates with temperature changes, which causes the optical output to fluctuate.

[0093] In one embodiment, this problem can be solved by making the reflectance Rwin of the reflective film 152 wavelength-dependent. Specifically, in the practical oscillation wavelength band of the LD chip 110, the sensitivity S of the light receiving element and the reflectance Rwin of the glass window have inverse wavelength dependences.

[0094] Fig. 18 is a diagram showing the wavelength dependence of sensitivity S of a Si photodiode, showing the characteristics of three photodiodes with different sensitivities.

[0095] The temperature coefficient of the LD's oscillation wavelength is assumed to be +0.064 nm / ℃, the actual temperature range is -5℃ to 85℃, and the individual variation of the oscillation wavelength is assumed to be ±2.5 to ±5 nm. In this case, the LD's oscillation wavelength λ can vary within a wavelength width range of 12 nm to 20 nm. For example, if the typical value (design value) of the oscillation wavelength is λ0, the oscillation wavelength range in actual use is λ0 ± (6 to 10) nm. Here, λ0 = 405 nm, and the actual oscillation wavelength band is 395 nm to 415 nm.

[0096] Consider the sensitivity slope α of the light receiving element. In this specification, the sensitivity slope α is defined as the rate of change in sensitivity S when the wavelength changes by 1 nm in the vicinity of the design wavelength λ0. The normalized sensitivity S norm (λ) is norm The sensitivity slope α is the normalized sensitivity S(λ)=S(λ) / S(λ0). norm The slope is (λ).

[0097] In the example of the Si photodiode in FIG. 18, the sensitivity slope α is +4% / nm for the low sensitivity one, +0.8% / nm for the medium sensitivity one, and +0.6% / nm for the high sensitivity one.

[0098] In order to cancel the wavelength dependency of the sensitivity of the light receiving element, it is sufficient that the slope for each wavelength of the ratio (Pf" / Po) of the power Pf" of the incident light L4 detected by the light receiving element 170 to the power Po of the external optical output L2 has a slope -α (% / nm) in the opposite direction to the sensitivity slope α (% / nm) of the light receiving element. Here, it is assumed that the relationship Pf"≫Pr' holds.

[0099] Here, since Pf″∝Pf′, it is sufficient that the slope of the power ratio Pf′ / Po for each wavelength has a slope in the opposite direction to the sensitivity S of the light receiving element. Pf' = Pf × Rwin Po = Pf × (1-Rwin) Therefore, Pf' / Po=Pf×Rwin / {Pf×(1-Rwin)} =Rwin / (1-Rwin) In other words, when the reflected / transmitted intensity ratio β=Rwin / (1-Rwin) has a slope opposite to the sensitivity slope α (% / nm) of the light receiving element, the fluctuation of the external optical output L2 can be suppressed.

[0100] In other words, when β(λ)×S(λ) is constant regardless of wavelength, that is, when β(λ)×S(λ)≒β(λ0)×S(λ0) holds, the fluctuation of the external optical output L2 can be suppressed. If you want to suppress the fluctuation of the external optical output L2 within ±5%, 0.95≦β(λ) / β(λ0)×S(λ) / S(λ0)≦1.05 It is sufficient if the following holds true.

[0101] Several examples of the design of the glass window 150 will now be described.

[0102] (Design example 1) 19 is a cross-sectional view of the reflective film 152 of the glass window 150 according to Design Example 1. The reflective film 152 has a structure in which two sets of five-layer bandpass filters 180 are stacked. The five-layer bandpass filters 180_1 and 180_2 each have a laminated structure of a λ / 8 layer 181, a λ / 4 layer 182, a λ / 2 layer 183, a λ / 4 layer 184, and a λ / 8 layer 185. The λ / 8 layer 181, the λ / 2 layer 183, and the λ / 8 layer 185 are SiO2 layers, and the λ / 4 layer 182 and the λ / 4 layer 184 are TiO2 layers. The SiO2 layer has a refractive index N1 value of 1.49, and the TiO2 layer has a refractive index N2 value of 2.82.

[0103] The thickness of each layer 18j (j=1 to 5) of each bandpass filter 180_i (i=1, 2) is represented as tij. ti1=ti5=λ REF / N1×(1 / 8+a·Ai) ti2=ti4=λ REF / N2×(1 / 4+b Bi) ti3=λ REF / N1×(1 / 2+c·Ci) Ai~Ci are the set values ​​for each set, and are fixed as A1=1, B1=1, C1=1, A2=-1, B2=-1, C2=-1. λ is a design parameter common to all sets. REF, a, b, and c were optimized.

[0104] Fig. 20 is a diagram showing the wavelength dependence of the optical characteristics of the reflective film 152 of Fig. 19. Fig. 20 shows the transmittance, reflectance, and reflection / transmission intensity ratio Rwin / (1-Rwin). The reflection / transmission intensity ratio Rwin / (1-Rwin) is a relative value normalized to be 1 at the wavelength λ0 (referred to as the reflection / transmission intensity ratio relative value). β norm (λ)=β(λ) / β(λ0) This shows that.

[0105] The adjusted parameters are as follows: λ REF =399.5nm, a=0.042,b=-0.004.c=-0.023

[0106] The reflectance Rwin at λ0=405 nm is 9.7%.

[0107] The sensitivity gradient α of the light receiving element 170 is set to 2.3% / nm. The normalized reflection / transmission intensity ratio β norm The graph of (λ) shows a reference slope line y0=1-α(λ-λ0) and two slope lines shifted by ±5% from this reference slope line. y - ={1-α(λ-λ0)}×0.95 y + ={1-α(λ-λ0)}×1.05

[0108] In this design example, the normalized reflection / transmission intensity ratio β norm (λ) is the slope of the two lines y + andy - In other words, in this wavelength band, the fluctuation in optical output can be suppressed to within 5%.

[0109] According to the reflective film 152 of FIG. 19, it is possible to adjust the slope rate of the relative value (normalized with respect to the value of the central wavelength (design wavelength)) of the reflection / transmission intensity ratio Rwin / (1-Rwin) in a range from near zero to a negative value with a maximum magnitude of about 3% / nm.

[0110] As a result, if the wavelength-dependent slope of the PD sensitivity relative value is 3% / nm or less, it is possible to find design parameters that have a wavelength dependence of the reflection / transmission intensity ratio relative value with an inverse slope rate that is the same magnitude but negative, making it possible to maintain the PD output value constant with an accuracy of ±5% even if there is wavelength fluctuation within the practical wavelength range.

[0111] (Design example 2) 21 is a cross-sectional view of the reflective film 152 of the glass window 150 according to Design Example 2. The reflective film 152 has a structure in which four sets of five-layer bandpass filters 180 are stacked.

[0112] As in design example 1, the thickness of each layer 18j (j=1 to 5) of each bandpass filter 180_i (i=1 to 4) is denoted by tij. ti1=ti5=λ REF / N1×(1 / 8+a·Ai) ti2=ti4=λ REF / N2×(1 / 4+b Bi) ti3=λ REF / N1×(1 / 2+c·Ci) Ai to Ci are the setting values ​​for each set and are fixed at the following values. A1=1, B1=1, C1=1 A2=-1, B2=-0.5, C2=-1 A3=1, B3=3, C3=1 A4=-1, B4=-1.25, C4=-1 Using this set value, λ, a design parameter common to all sets, REF , a, b, and c were optimized.

[0113] Fig. 22 is a diagram showing the wavelength dependence of the optical characteristics of the reflective film 152 of Fig. 21. Fig. 22 shows the transmittance, reflectance, and normalized reflection / transmission intensity ratio β norm(λ) is shown.

[0114] The adjusted parameters are as follows: λ REF =395nm, a=0.03,b=-0.07.c=-0.0055

[0115] The reflectance Rwin at λ0=405 nm is 9.5%.

[0116] The sensitivity slope α of the light receiving element 170 is set to 2.25% / nm. The normalized reflection / transmission intensity ratio β norm The graph of (λ) shows a reference slope line y0=1-α(λ-λ0) and two slope lines shifted by ±5% from this reference slope line. y - ={1-α(λ-λ0)}×0.95 y + ={1-α(λ-λ0)}×1.05

[0117] In this design example, the normalized reflection / transmission intensity ratio β norm (λ) is the slope of the two lines y + andy - In other words, in this wavelength band, the fluctuation in optical output can be suppressed to within 5%.

[0118] According to the reflective film 152 of FIG. 21, it is possible to adjust the slope rate of the relative value (normalized by the value of the central wavelength) of the reflection / transmission intensity ratio Rwin / (1-Rwin) in a range from near zero to a negative value with a maximum magnitude of about 4% / nm.

[0119] As a result, if the wavelength-dependent slope of the PD sensitivity relative value is 4% / nm or less, it is possible to find design parameters that have a wavelength dependence of the reflection / transmission intensity ratio relative value with an inverse slope rate that is the same magnitude but negative, making it possible to maintain the PD output value constant with an accuracy of ±5% even if there is wavelength fluctuation within the practical wavelength range.

[0120] 19 and 21, the smaller Rwin is, the larger the lower limit of the negative value of the gradient range of the reflection / transmission intensity ratio relative value that can be designed becomes, and the narrower the designable range becomes. From this point of view, therefore, it is preferable to set the reflectance Rwin ≧ 3%.

[0121] The embodiments merely illustrate the principles and applications of the present invention, and many modifications and changes in arrangement are permitted to the embodiments without departing from the spirit of the present invention defined in the claims. [Explanation of symbols]

[0122] 100 Semiconductor laser device 110 LD chip 112 Front reflective film 114 Back reflective film 114a Base layer 114b Dielectric multilayer film 116 Absorbing Layer 116a SiO2 layer 116b High refractive index layer 116c termination layer 116d metal layer 120 Submount 130 Metal stem 132 Slope 134 Reference plane in contact with the bottom surface of the submount 140 Metal Cap 150 Glass Window 152 Reflective film 160 Leads 170 Photodetector 172 Photosensitive surface 180 Bandpass Filter S1 front end S2 Rear end face L1 Front light output L2 External optical output L3 Reflected forward light output L4 incident light L5 rear light output L6 incident light

Claims

1. an edge-emitting semiconductor laser chip that radiates a forward optical output from a forward end face; a glass window provided on the front end surface side of the semiconductor laser chip and configured to reflect a portion of the forward optical output; a light receiving element provided on a rear end surface side of the semiconductor laser chip so as to receive a reflected forward light output, which is the forward light output reflected by the glass window; Equipped with a sensitivity of said light receiving element and a reflectance of said glass window having an inverse wavelength dependency in the practical oscillation wavelength band of said semiconductor laser chip;

2. 2. The semiconductor laser device according to claim 1, wherein in an actual oscillation wavelength band of the semiconductor laser chip, a reflection / transmission intensity ratio β(λ)=Rwin(λ) / (1−Rwin(λ)) has a wavelength dependency that is inverse to that of the sensitivity S(λ), where S(λ) is the sensitivity of the light receiving element and Rwin(λ) is the reflectance of the glass window.

3. A wavelength within the practical oscillation wavelength band is defined as λ ０ , the normalized sensitivity is S ｎｏｒｍ (λ)=S(λ) / S(λ ０ ), the normalized sensitivity S ｎｏｒｍ When (λ) has a slope α, Normalized reflected / transmitted intensity ratio β ｎｏｒｍ (λ)=β(λ) / β(λ ０ ) is a straight line {1-α(λ-λ)} in the practical oscillation wavelength band. ０ )}×0.95 and {1-α(λ-λ ０ 3. The semiconductor laser device according to claim 2, wherein the semiconductor laser device is sandwiched between a thickness of 1.0 mm and a thickness of 1.0 mm.

4. 3. The semiconductor laser device according to claim 1, wherein Rwin is greater than or equal to 3%.

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