ATHERMAL LASER OPTICS MADE OF PLASTICS

Abstract

The invention relates to an athermalized device for generating laser radiation that is focused in a focal point, comprising a lens and a plastic housing and a passive adjustment system for adjusting the object distance S1. The passive adjustment device has an effective coefficient of thermal expansion (I)

[00001]${\alpha }_V=\frac{s_1}{f}.Math.\left[{\alpha }_L-\frac{1}{n-1}.Math.\left(\frac{\partial n}{\partial \lambda }.Math.\frac{d\lambda }{\mathrm{dT}}+\frac{\partial n}{\partial T}\right)\right]+{\alpha }_2.Math.\left(1-\frac{s_1}{f}\right).$

Claims

1. A device for generating laser radiation concentrated to a focal point, comprising a laser diode having a central wavelength λ and a temperature dependence of the central wavelength $\frac{d\lambda }{\mathrm{dT}},$ a lens having a focal length f, made from a first plastic having a refractive index n, a wavelength dependence of the refractive index $\frac{\partial n}{\partial \lambda },$ a temperature dependence of the refractive index $\frac{\partial n}{\partial T}$ and a coefficient of thermal expansion α.sub.L, wherein the lens results in an optical image of the laser radiation at the focal point and the optical image has an object distance s.sub.1, a housing made of a second plastic having a coefficient of thermal expansion α.sub.2 a passive adjustment device having an effective coefficient of thermal expansion α.sub.V for adjusting the object distance s.sub.1, wherein the effective coefficient of thermal expansion is ${\alpha }_V=\frac{s_1}{f}.Math.\left[{\alpha }_L-\frac{1}{n-1}.Math.\left(\frac{\partial n}{\partial \lambda }.Math.\frac{d\lambda }{\mathrm{dT}}+\frac{\partial n}{\partial T}\right)\right]+{\alpha }_2.Math.\left(1-\frac{s_1}{f}\right).$

2. The device as claimed in claim 1, wherein the optical image has an image distance s.sub.2 greater than five times the object distance 5.Math.s.sub.1.

3. The device as claimed in claim 1, wherein the laser diode emits laser radiation with a beam quality factor M.sup.2<1.3.

4. The device as claimed in claim 1, wherein the focal point takes the form of an image beam waist having a beam quality factor M.sup.2<1.3 and/or in that the lens takes the form of an aspherical lens.

5. The device as claimed in claim 1, wherein the effective coefficient of thermal expansion of the adjustment device α.sub.V>100 μm/(m.Math.K).

6. The device as claimed in claim 1, wherein the adjustment device has a first stretcher having a coefficient of thermal expansion α.sub.a>50 μm/(m.Math.K) and a second stretcher that counteracts the first stretcher and has a coefficient of thermal expansion α.sub.b<20 μm/(m.Math.K), and in that the first stretcher consists of a third plastic.

7. The device as claimed in claim 1, wherein it has been athermalized at least within a temperature range from −40° C. to +85° C. and/or in that the third plastic has a glass transition temperature above that temperature range.

8. The use of a device as claimed in claim 1 for illuminating a sample for the purpose of conducting a measurement of a physical or technical parameter.

Description

SUMMARY OF THE INVENTION

[0014] It is an object of the invention to specify an inexpensive device for generating laser radiation concentrated to a focal point, which is operable over a wide temperature range.

[0015] The device of the invention for generating laser radiation concentrated to a focal point is producible inexpensively and operable over a wide temperature range. It is advantageous that only a single lens made of a plastic is required for the device. It is possible to dispense with having to use a costly glass lens or achromatic lens combination.

[0016] The object is achieved by an inexpensive device for generating laser radiation concentrated to a focal point as claimed in claim 1, and use as claimed in claim 8.

[0017] The device (1) for generating laser radiation concentrated to a focal point (6) comprises

a laser diode (3) having a central wavelength λ and a temperature dependence of the central wavelength

[00002]$\frac{d\lambda }{\mathrm{dT}},$

a lens (8) having a focal length f, made of a first plastic having a refractive index n, a wavelength dependence of the refractive index

[00003]$\frac{\partial n}{\partial \lambda },$

a temperature dependence of the refractive index

[00004]$\frac{\partial n}{\partial T}$

and a coefficient or thermal expansion α.sub.L, wherein the lens results in an optical image of the laser radiation at the focal point (9) and the optical image has an object distance s.sub.1,
a housing (11) made of a second plastic having a coefficient of thermal expansion α.sub.2
a passive adjustment device (12) having an effective coefficient of thermal expansion α.sub.V for adjustment of the object distance s.sub.1, wherein the effective coefficient of thermal expansion is

[00005]${\alpha }_V=\frac{s_1}{f}.Math.\left[{\alpha }_L-\frac{1}{n-1}.Math.\left(\frac{\partial n}{\partial \lambda }.Math.\frac{d\lambda }{\mathrm{dT}}+\frac{\partial n}{\partial T}\right)\right]+{\alpha }_2.Math.\left(1-\frac{s_1}{f}\right).$

[0018] The focal point may, but need not, lie on the optical axis. The focal point, for the purposes of geometric radiation optics, may be considered as a point of intersection of the convergent light rays diffracted by the lens. From the point of view of wave optics, the focal point may be a Gaussian beam waist, i.e. the waist of a Gaussian beam.

[0019] The first plastic may advantageously be a transparent plastic, for example polystyrene (PS), polyethylene terephthalate (PET), polyallyldiglycol carbonate (PADC), polycarbonate (PC), polymethylmethacrylate (PMMA), a cycloolefin polymer (COP) or a synthetic resin, for example phenolic resin, urea resin, melamine resin, polyester resin or epoxy resin.

[0020] The second plastic may advantageously be, for example, PS, PVC, PET, PADC, PC, PMMA, PEEK, PE, PP, COP, polysulfone (PSU) or a synthetic resin, for example phenolic resin, urea resin, melamine resin, polyester resin or epoxy resin. The second plastic may have been modified to absorb light and/or colored with the aid of admixtures, for example carbon or dye. Likewise advantageously, the second plastic may have been fiber-reinforced.

[0021] The device may have a transfer matrix M=M.sub.s2M.sub.LM.sub.s1, where the indices may each range from 0 to 1. It may be the case here that

[00006]$M_{s2}=\left(\begin{array}{cc}1& s_2\\ 0& 1\end{array}\right)$

is the transfer matrix of the free beam after the lens,

[00007]$M_{s1}=\left(\begin{array}{cc}1& s_1\\ 0& 1\end{array}\right)$

is the transfer matrix of the free beam before the lens and

[00008]$M_L=\left(\begin{array}{cc}1& 0\\ -1/f& 1\end{array}\right)$

is the transfer matrix of the lens, reported here in the customary approximation of a thin lens. The image distance s.sub.2 may have the temperature dependence α.sub.2 of the housing material. This can mean that a target focal point position moves, or is intended to move, along the optical axis with the thermal expansion of the housing. The focal length f of the lens may be dependent on three thermal influencing parameters. On the basis of the coefficient of thermal expansion α.sub.L in the event of a change in temperature, there can be a change in the shape of the lens, which alters the focal length. As a result of the temperature dependence of the refractive index

[00009]$\frac{\partial n}{\partial T},$

there can be a change in the focal length. Moreover, the wavelength of the laser, i.e. the central wavelength of the laser radiation, can change with the change in temperature owing to the temperature dependence

[00010]$\frac{d\lambda }{\mathrm{dT}}.$

On account of the wavelength dependence of the refractive index

[00011]$\frac{\partial n}{\partial \lambda }$

of the lens, there may thus likewise be a change in the focal length of the lens.

[0022] First-order development of the transfer matrix to a series gives

[00012]$M\left(T\right)=M\left(T_0\right)+\Delta T.Math.\frac{d}{dT}M$

with a reference temperature T.sub.0 and a temperature variance ΔT from the reference temperature. The derivation of the transfer matrix is

[00013]$\frac{d}{dT}M=\frac{\partial M}{\partial s_1}.Math.\frac{d{s}_1}{dT}+\frac{\partial M}{\partial s_2}.Math.\frac{d{s}_2}{dT}+\frac{\partial M}{\partial f}.Math.f.Math.\left[\frac{-1}{n-1}.Math.\left(\frac{\partial n}{\partial T}+\frac{\partial n}{\partial \lambda }.Math.\frac{d\lambda }{dT}\right)+{\alpha }_L\right]$

with the effective coefficient of thermal expansion of the adjustment device

[00014]$\frac{d{s}_1}{dT}={\alpha }_V$$\mathrm{and}$$\frac{d{s}_2}{dT}={\alpha }_2$

[0023] Under the condition that the zeroth index of the transformed beam vector in the plane of the image at the reference temperature and any angles φ of a beam relative to the optical axis within an aperture of the optical system is zero,

[00015]${\left[M\left(T_0\right).Math.\left(\begin{array}{c}0\\ \phi \end{array}\right)\right]}_0=0$

the imaging condition at the reference temperature is obtained:

[00016]$s_2\left(T_0\right)=\frac{s_1\left(T_0\right)}{\frac{s_1\left(T_0\right)}{f}-1}$

[0024] The imaging condition should likewise be applicable at a different temperature

[00017]${\left[M\left(T_0+\Delta T\right).Math.\left(\begin{array}{c}0\\ \phi \end{array}\right)\right]}_0=0$

and in the imaging plane that has now moved

[00018]$s_2\left(T\right)=s_2\left(T_0\right).Math.\left(1+{\alpha }_2.Math.\Delta T\right)$

For this purpose, it must be the case that

[00019]${\left[\frac{d}{dT}M.Math.\left(\begin{array}{c}0\\ \phi \end{array}\right)\right]}_0=0$

[0025] The calculation of the above expression leads to the required effective coefficient of thermal expansion of the adjustment device

[00020]${\alpha }_V=\frac{s_1}{f}.Math.\left[{\alpha }_L-\frac{1}{n-1}.Math.\left(\frac{\partial n}{\partial \lambda }.Math.\frac{d\lambda }{dT}+\frac{\partial n}{\partial T}\right)\right]+{\alpha }_2.Math.\left(1-\frac{s_1}{f}\right)$

This is envisaged in accordance with the invention.

[0026] For example, the lens may consist of a first plastic PC with

[00021]${\alpha }_L=70.Math.{10}^{-6}/K,n=1.579,\frac{\partial n}{\partial T}=107.Math.{10}^{-6}/K\mathrm{and}\frac{\partial n}{\partial \lambda }=-0.07/\mathrm{.Math.m}.$

The lens may have a focal length f=5 mm, and it may be that an object distance s.sub.1=7 mm and an image distance s.sub.2=17.5 mm. The housing may consist of a second plastic, a blackened PC with α.sub.2=70.Math.10.sup.−6/K. The laser diode may have a wavelength of 650 nm with a temperature dependence

[00022]$\frac{d\lambda }{dT}=0.30n\text{m/K}.$

In this example, according to the invention, it would be necessary to provide an effective coefficient of thermal expansion of the adjustment device α.sub.V=380.Math.10.sup.−6/K.

[0027] In the case of a thick lens, the object distance s.sub.1 and the image distance s.sub.2 may each be reported in a known manner with regard to the object-side/image-side main plane of the lens. In the approximation of a thin lens, it is possible to assume a common main plane since the object-side and image-side main planes approximately coincide.

[0028] The device of the invention, in an advantageous embodiment, may be characterized in that the optical image has an image distance s.sub.2 greater than five times the object distance 5.Math.s.sub.1. The imaging operation may thus be an enlargement. The device of the invention may likewise advantageously be characterized in that the optical image has an image distance s.sub.2 less than twenty times the object distance 20.Math.s.sub.1. The image distance may be, for example, between 5 mm and 25 mm.

[0029] The laser diode may advantageously be a single-mode laser diode. It can emit laser radiation with a beam quality factor M.sup.2<1.3. The laser diode may advantageously be selected from a wavelength range between 600 nm and 800 nm. The central wavelength of the laser diode, depending on the temperature, may change, for example, by 0.25 . . . 0.40 nm/K. The laser radiation may be linear-polarized.

[0030] The device of the invention may advantageously be characterized in that the focal point takes the form of an image beam waist having a beam quality factor M.sup.2<1.3. Advantageously, the lens may take the form of an aspherical lens. It may have one or two aspherical faces. As a result, it is possible to obtain the beam quality factor of the laser diode. The lens may be a rotationally symmetrical lens. Alternatively, it is also possible to use an astigmatic lens. In this case, the beam path may be specified in a meridional plane. In the case of a cylindrical lens, this may be the active plane. In general, in the case of an astigmatic lens, it is possible to use the meridional plane having the shortest focal length. The laser diode may have an astigmatism that can be corrected with an astigmatic lens.

[0031] The effective coefficient of thermal expansion of the adjustment device may be α.sub.V>100 μm/(m.Math.K).

[0032] The adjustment device may comprise a first stretcher having a coefficient of thermal expansion α.sub.a>50 μm/(m.Math.K). Advantageously, the coefficient of expansion of the first stretcher may be greater than the coefficient of thermal expansion α.sub.2 of the housing.

[0033] Advantageously, the length l.sub.a of the first stretcher may be greater than the object length s.sub.1. It is also possible for the adjustment device to comprise a second stretcher (14) that counteracts the first stretcher and has a lower coefficient of thermal expansion α.sub.b<α.sub.a. It may advantageously be the case that α.sub.b<20 μm/(m.Math.K). The second stretcher may have a length l.sub.b. The length l.sub.b may be shorter than the length l.sub.a by about the distance s.sub.1. The first stretcher may consist of a third plastic. The second stretcher may consist of a fourth plastic, a glass, a ceramic, for example Al.sub.2O.sub.3, ZrO or a metal. The combination of the two structures can achieve a coefficient of thermal expansion above that of the first stretcher. The effective coefficient of thermal expansion of the adjustment device may be

[00023]${\alpha }_V=\frac{l_a.Math.{\alpha }_a-.Math.l_b.Math.{\alpha }_b}{s_1}.$

The first stretcher and/or second stretcher may be in tubular form.

[0034] The third plastic may advantageously, for example, be PS, PVC, PET, PADC, PC, PMMA, PEEK, PE, PP, PSU, COP or a synthetic resin, e.g. phenolic resin, urea resin, melamine resin, polyester resin or epoxy resin. The third plastic may have been modified to absorb light and/or colored with the aid of admixtures, for example carbon or dye.

[0035] The fourth plastic may advantageously, for example, be liquid-crystal polymer (LCP), PS, PVC, PET, PADC, PC, PMMA, PEEK, PE, PP, PSU COP or a synthetic resin, e.g. phenolic resin, urea resin, melamine resin, polyester resin or epoxy resin. The fourth plastic may have been modified to absorb light and/or colored with the aid of admixtures, for example carbon or dye. Likewise advantageously, the fourth plastic may have been reinforced with fibers and/or filled with ceramic particles. In this way, it is possible to lower the coefficient of thermal expansion α.sub.b compared to the starting material. Advantageous fibers may be aramid, glass or carbon fibers.

[0036] The device of the invention may have been athermalized at least within a temperature range from −40° C. to +85° C. Advantageously, the third plastic may have a glass transition temperature above the maximum temperature of the temperature range specified. Particularly advantageously, the first and second plastics may also have a glass transition temperature above the maximum temperature of the temperature range specified.

[0037] It may be advantageous to use a device (1) of the invention for illuminating a sample for the purpose of conducting a measurement of a physical or technical parameter. The parameter to be measured may, for example, be the rotation of the plane of polarization of the light, the absorption of a sample at a particular wavelength of light, the scatter of the light at inhomogeneities or particles present in the sample, the size or number of particles in a sample, or the intensity of fluorescence radiation of a sample.

[0038] Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.

SOLE FIGURE

[0039] The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawing which is given by way of illustration only, and thus, is not limitive of the present invention, and wherein the sole FIGURE illustrates a first working example.

WORKING EXAMPLES

[0040] The invention is elucidated hereinafter by a working example.

[0041] The device (1) shown for generating laser radiation concentrated to a focal point (9) comprises

a laser diode (3) having a central wavelength λ and a temperature dependence of the central wavelength

[00024]$\frac{d\lambda }{dT},$

a lens (8) having a focal length f, made of a first plastic having a refractive index n, a wavelength dependence on the refractive index

[00025]$\frac{\partial n}{\partial \lambda },$

a temperature dependence of the refractive index

[00026]$\frac{\partial n}{\partial T}$

and a coefficient of thermal expansion α.sub.L, wherein the lens results in an optical image of the laser radiation at the focal point (9) and the optical image has an object distance s.sub.1,
a housing (11) made of a second plastic having a coefficient of thermal expansion α.sub.2
a passive adjustment device (12) having an effective coefficient of thermal expansion α.sub.V for adjustment of the object distance s.sub.1, wherein the effective coefficient of thermal expansion is

[00027]${\alpha }_V=\frac{s_1}{f}.Math.\left[{\alpha }_L-\frac{1}{n-1}.Math.\left(\frac{\partial n}{\partial \lambda }.Math.\frac{d\lambda }{dT}+\frac{\partial n}{\partial T}\right)\right]+{\alpha }_2.Math.\left(1-\frac{s_1}{f}\right).$

[0042] The device has an optical axis 2. The laser diode comprises a laser diode housing 4, in which there is disposed a laser diode chip 5. The front of the laser diode housing 4 is equipped with a window from which the laser radiation 7 exits. The electrical contacts of the laser diode are made by means of contact pins 6. The laser diode emits a bundle of rays 7 which is concentrated to a focal point 9 by means of the lens 8. The focal point is represented as a Gaussian beam waist in the diagram. The focal point is within a measurement volume 10 that may take the form of a measurement chamber connected to or integrated into the housing 11.

[0043] Also present is an adjustment device (12) comprising a first stretcher (13) having a coefficient of thermal expansion α.sub.a>50 μm/(m.Math.K) and a second stretcher (14) that counteracts the first stretcher and has a coefficient of thermal expansion α.sub.b<20 μm/(m.Math.K). The first stretcher consists of a third plastic.

[0044] The measurement volume 10 may be intended to accommodate a sample for the purpose of conducting a measurement of a physical or technical parameter. The parameter to be measured may, for example, be the rotation of the plane of polarization of the light, the absorption of the sample at a particular wavelength of light, the scatter of the light at inhomogeneities or particles present in the sample, the size or number of particles in the sample, or the intensity of fluorescence radiation from the sample. The light scatter to be measured may be classical scatter, Mie scatter or Rayleigh scatter.

[0045] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are to be included within the scope of the following claims.