Semiconductor laser device

Abstract

To provide an internal PD monitor-type semiconductor laser device in which long-term output stability 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. Power Pr' of rearward optical output L6 that is emitted from the rearward end face S2 of the LD chip 110 and that is incident on the light-receiving element 170 is 1 / 3 or less of power Pf" of the reflected forward optical output L4 that is incident on the light-receiving element 170.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. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5319397 Summary of the Invention [Problem to be solved by the invention]

[0005] The present inventors have studied LDs with internal PD monitors and have come to recognize the following problems.

[0006] 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)

[0007] Therefore, the ratio of Pr to Pf, Pr / Pf, is expressed by equation (1). Pr / Pf=√Rf / √Rr×(1-Rr) / (1-Rf) …(1)

[0008] The LD facets in the GaN / InGaN wavelength band lack optical stability in the dielectric reflective film that determines the reflectance characteristics, the interface between the dielectric reflective film and the semiconductor, and the state of the semiconductor in the vicinity, and optical deterioration of the facet film structure is unavoidable after long-term operation. As a result of this optical deterioration, the reflectances Rf and Rr fluctuate. If Pr / Pf deviates from the designed wavelength dependency (PD sensitivity compensation slope characteristic as in Patent Document 1), the external optical output Po deviates from the allowable range of the specified APC specification.

[0009] This problem can occur even under operating conditions where the environmental temperature is kept constant and the oscillation wavelength is kept constant, and is common to the internal PD monitor method in which the majority of the light received by the PD comes from the backward optical output from the LD, regardless of whether the film structure has an appropriately controlled wavelength dependence of Pr / Pf.

[0010] The present disclosure has been made in consideration of the above problems, and an exemplary purpose of an embodiment of the present disclosure is to provide a semiconductor laser device with an internal PD monitor system that improves long-term output stability. [Means for solving the problem]

[0011] An aspect 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 provided on the front end face side of the semiconductor laser chip and reflecting a part of the forward optical output, and a light-receiving element provided on the rear end face side of the semiconductor laser chip so as to receive the reflected forward optical output, which is the forward optical output reflected by the glass window. The power of the rear optical output radiated from the rear end face of the semiconductor laser chip that is incident on the light-receiving element is 1 / 3 or less of the power of the reflected forward optical output that is incident on the light-receiving element.

[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, it is possible to improve the long-term output stability of a semiconductor laser device using an internal PD monitor method. [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. 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 provided on the front end face side of the semiconductor laser chip and reflecting a part of the forward optical output, and a light-receiving element provided on the rear end face side of the semiconductor laser chip so as to receive the reflected forward optical output, which is the forward optical output reflected by the glass window. The power of the rear optical output radiated from the rear end face of the semiconductor laser chip that is incident on the light-receiving element is ⅓ or less of the power of the reflected forward optical output that is incident on the light-receiving element.

[0017] 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 three times or more 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 of the semiconductor laser chip to the power of the forward optical output is required to be constant. In contrast, the power of the forward optical output is directly monitored, so that the optical output from the front is dominant and is less susceptible to the influence of the optical output from the rear, eliminating the need to take care to keep the ratio of the power of the rear optical output of the semiconductor laser chip to the power of the forward optical output constant. 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.

[0018] 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.

[0019] 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 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 expected package sizes and semiconductor laser chips with various specifications.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] In one embodiment, the semiconductor laser chip may include an absorption layer formed on a rear reflector film formed on the rear facet, which can reduce the power of the rear optical output incident on the photodetector.

[0025] In one embodiment, the absorbing layer may include a high refractive index layer or may include a metal film with a large extinction coefficient.

[0026] In one embodiment, the transmittance of the absorbing layer may be 1% or less.

[0027] (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.

[0028] 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.

[0029] (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.

[0030] 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.

[0031] 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).

[0032] 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.

[0033] 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).

[0034] The above is the configuration of the semiconductor laser device 100A. Next, the operation of the device will be described.

[0035] 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.

[0036] 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.

[0037] 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)

[0038] 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)

[0039] 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)

[0040] 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)

[0041] 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.

[0042] "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.

[0043] The above is the operation of the semiconductor laser device 100A. Next, its advantages will be described.

[0044] 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.

[0045] By substituting equations (1) to (5) into equation (6), we obtain equation (7). Pdet=Pf”+Pr'=Pf×Rwin×ηf+Pr×ηr …(7)

[0046] 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)

[0047] 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.

[0048] 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.

[0049] 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.

[0050] According to this embodiment, even if fluctuations in the LD characteristics (changes in Pr / Pf due to facet degradation), such as fluctuations in the LD facet reflectivity Rf or Rf, occur during long-term power-on operation, the ratio of the package's external optical output L2 to the internal PD monitor value is stably maintained, enabling accurate APC operation.

[0051] (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.

[0052] 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.

[0053] (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).

[0054] 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".

[0055] (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 Ls is the distance from the center of the light receiving surface 172 of the light receiving element 170 in the direction along the inclined surface 132, and is the distance (shift amount) between 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: Width of the light receiving surface 172 of the light receiving element 170

[0056] The following dimensions were used. D=0.3mm d=0.3mm α=13° Lf=0.5mm Lr=0.3mm,0.5mm,0.7mm Lc=0.6mm,0.8mm,1.0mm,1.2mm,1.5mm,2.0mm H=0.5mm L=0.5mm

[0057] The forward optical output L1 and the backward optical output L5 emitted from the LD chip 110 are assumed to have a Gaussian profile, with the beam's vertical (Z-direction) divergence angle θv being 22° and the beam's horizontal (Y-direction) divergence angle θh being 9°.

[0058] Figure 7 shows the power Pf",Pr' of 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 set to 1, on a logarithmic axis. The horizontal axis shows the position of the light receiving element 170 (Ls in Figure 6).

[0059] Each figure 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.

[0060] 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.

[0061] Figure 8 is a diagram showing the powers Pf” and Pr’ of the light incident on the light receiving element 170 when Rwin = 3%. The four graphs correspond to Rr = 90%, 99%, 99.5%, and 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.

[0062] 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.

[0063] Also, when reducing the reflectance Rwin of the glass window 150, the reflectance Rr of the rear end face S2 at which the shift amount Ls of the light receiving element 170 can be set to 0 increases.

[0064] 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.

[0065] · 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.

[0066] 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.

[0067] 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 to Rwin = 1%, the condition Pf” > 0.005 to ensure the output current of the PD cannot be satisfied.

[0068] The following findings can be obtained from the comparison of FIGS. 7 to 10. When the reflectance Rwin of the glass window 150 is reduced, the shift amount Ls of the light receiving element 170 required to satisfy the design conditions increases. When the reflectance Rwin of the glass window 150 is reduced, the upper limit of the shift amount of the light receiving element 170 also becomes smaller. 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.

[0069] 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.

[0070] 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.

[0071] (Design of glass window 150) Next, the reflectance Rwin of the glass window 150 will be described.

[0072] 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)

[0073] 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)

[0074] 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.

[0075] 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

[0076] This antireflection film 154 can achieve a reflectance of 0.1% or less in the practical wavelength band of 395 to 415 nm. Therefore, the reflectance Rwin can be designed by taking into account only the inner reflective film 152.

[0077] 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%.

[0078] 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 Rd, the reflective film 152 may be designed so as to satisfy Ro = Ri = Rd / 2.

[0079] Also, when Ro ≠ Ri and Ro is fixed in advance to a certain value, by calculating the inverse 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 Rd, the reflective 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 fixed in advance to a certain value instead of Ro, by calculating the inverse 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 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 Rd which is less 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.

[0080] (Design of the Reflective Film of the LD Chip 110) 12 is a cross-sectional view of an embodiment of the LD chip 110. As described above, a high reflectance Rr is required for the rear reflection film 114 of the LD chip 110. The rear reflection film 114 can be formed of a dielectric multilayer film.

[0081] 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 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).

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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]

[0088] 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 S1 front end S2 Rear end face L1 Front light output L2 external light output L3 reflects front 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 13. A semiconductor laser device, comprising: a semiconductor laser chip having a rear end surface and a rear light output radiated from the rear end surface of the semiconductor laser chip, the rear light output being incident on the light receiving element; and a power of the rear light output being incident on the light receiving element, the rear light output being equal to or less than 1 / 3 of the power of the reflected forward light output.

2. 2. The semiconductor laser device according to claim 1, wherein the power of the backward 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.

3. 2. The semiconductor laser device according to claim 1, wherein the reflectance Rwin of said glass window is 2% or more.

4. 4. The semiconductor laser device according to claim 1, wherein the reflectance of a rear reflection film formed on the rear end face of the semiconductor laser chip is 99.5% or more.

5. 5. The semiconductor laser device according to claim 4, wherein the reflectance of the rear reflection film formed on the rear end face of the semiconductor laser chip is 99.9% or more.

6. 4. The semiconductor laser device according to claim 1, wherein the light receiving element is arranged offset with respect to the semiconductor laser chip when the semiconductor laser device is viewed along the emission direction of the forward light output.

7. 4. The semiconductor laser device according to claim 1, wherein the semiconductor laser chip further comprises an absorption layer formed on a rear reflection film formed on the rear end face.

8. 8. The semiconductor laser device according to claim 7, wherein the transmittance of said absorption layer is 1% or less.

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