Meas. Sci. Technol. 10 (1999) 146–151. Printed in the UK
`
`PII: S0957-0233(99)97872-4
`
`The suitability of sapphire for laser
`windows
`
`B S Patel† and Z H Zaidi‡
`† Defence Science Centre, Metcalfe House, Delhi 110 054, India
`‡ Department of Physics, Jamia Millia Islamia, New Delhi 110 025, India
`
`Received 28 September 1998, in final form 16 November 1998, accepted for publication
`26 November 1998
`
`Abstract. The present paper deals with the experimental measurement of absorption
`coefficients using (i) the differential dual-beam spectro-photometry method and (ii) an
`intra-cavity optical loss measurement technique. The absorption coefficient is measured
`experimentally in the region from the visible to the near IR. Criteria for choosing sapphire
`windows of optimum thickness, when they are subjected to a pressure difference of 1 atm, to
`avoid fracture or significant optical distortion of the laser beam are studied. Intra-cavity
`optical losses in such windows for Brewsterization and skew-angle inclusions are estimated.
`The study relates to various laser wavelengths, namely 337.1 nm, 448.0 nm, 632.8 nm,
`1.15 μm, 1.315 μm, 2.7 μm and 3.8 μm.
`
`Keywords: optical transmission, laser window material, diagnostics
`
`1. Introduction
`
`Sapphire is known as an optical material of outstanding
`merits. Nonetheless, its high cost, scarcity in pure quality and
`crystalline impurities have limited its use in the optical and
`electro-optical industry. Modern technology has now made
`it possible to obtain bulk sapphire crystals of excellent purity.
`Consequently, sapphire is now emerging as a challenger
`to the conventional optical materials like fused silica and
`boro-silicate glasses in the visible region. Particularly for
`high-power laser optics and for military applications, for
`which reliability and ruggedness become important deciding
`factors, in addition to the optical quality, sapphire is most
`likely to replace the existing optical materials. The feasibility
`of utilizing high-power lasers for directed energy weapons
`has already been established and their deployment for actual
`engagement in the next century is being projected. For this
`purpose the earlier candidate laser had been the DF laser
`(lasing at 3.8 μm) but the present analysis reveals that, for all
`such military applications, the best suited laser would be the
`chemical oxy-iodine laser (COIL) operating at 1.315 μm.
`For such a critical role, it is imperative to investigate the
`suitability of sapphire as a laser window material in the UV
`and visible ranges and particularly in the near-IR regions.
`Also, it is noteworthy that the refractive index (Maliston
`1962) of sapphire is around 1.76 (see table 1); hence it is
`possible to fabricate a few specific laser mirrors by using a
`combination (Patel and Ben Bouzid 1984) of plane-parallel
`flats, thereby eliminating the multi-layer dielectric coatings
`having an inherently lower power handling capacity, limited
`due to the damage to the coating caused by the laser intensity.
`
`0957-0233/99/030146+06$19.50 © 1999 IOP Publishing Ltd
`
`Table 1. Physical constants of sapphire relevant to the present
`study.
`Moh 9
`Hardness
`(5.0–5.6) × 107 psi
`Young’s modulus
`−0.02
`Poisson’s ratio
`65 000–100 000 psi
`Modulus of rupture
`Indices of refraction μe = 1.760
`μo = 1.768
`(sodium D-line birefringence)
`
`2. The physical suitability of sapphire material
`
`The most important consideration in selecting a material for
`a laser window is that its bulk absorption coefficient must
`be very low (Patel and Charan 1975, Patel 1990). High
`absorption not only reduces the laser power but also produces
`undesirable heating of the optical components which may
`eventually result in their thermal runaway. The surfaces of
`the optical components can also contribute substantially to the
`optical losses, but they are largely dependent on the polishing
`techniques and can be suitably controlled.
`In addition,
`it is desirable that an optical material has the following
`properties:
`(i) high mechanical strength,
`(ii) chemical
`inertness and high resistance to environment-related causes
`of deterioration, (iii) high-temperature stability, (iv) wear
`and abrasion resistance, (v) zero porosity, (vi) high thermal
`conductivity and (vii) high electrical resistivity. Sapphire
`is really unique in possessing the aforesaid qualifications.
`Being one of the hardest materials, sapphire can only be
`scratched, besides by itself, by a few substances such as
`diamond and cubic boron nitride. It is chemically inert in
`almost everything at room temperature and can be attacked
`only by chemicals like hydrofluoric acid at temperatures
`
`ASML 1116
`
`

`
`Sapphire for laser windows
`
`Figure 1. The absorption coefficient (α) and refractive index for the ordinary ray (μ) of sapphire as functions of the wavelength (λ).
`Sources of data for plotting absorption coefficient curve are as follows: ⊕, Deutsch (1975); +, Harrington et al (1976); (cid:2) Thomos et al
`(1988); Innocenzi et al (1990); and in the present investigations: (cid:3) using spectrophotometer, (cid:4)(cid:5) using a He–Ne laser at 632.8 nm and ⊗
`using a He–Ne laser at 1.15 μm.
`
`above 300 ◦C. This property allows one to obtain ultra-clean
`sapphire components through the use of chemical reagents.
`It is for this reason that sapphire optics can be used for HF
`and DF lasers. Other relevant qualities are obvious from the
`properties listed in table 1. Its energy band gap is 10 eV, which
`is one of the largest for the oxide crystals. Hence it exhibits
`remarkable transmission from 150 nm in the vacuum-UV
`region to 6.0 μm in the middle IR. However, its optical
`suitability for laser windows in this transmission range will
`be investigated in this study.
`Sapphire is an optically negative uniaxial crystal in
`the visible region and exhibits anisotropy in its physical
`properties. Generally most Czochralski-grown sapphire
`crystals have the C axis (or the optical axis) inclined at an
`angle of 60◦ to the growth axis. The results of Innocenzi
`et al (1990) on absorption in sapphire at room temperature
`indicate that absorption coefficients of 60◦-oriented crystals
`are low relative to those of 0◦-oriented crystals for photons
`having energies in the range 5–9 eV. Hence, if the use of
`optical components with 0◦ orientation in order to avoid
`the undesirable birefringence is preferred, they should be
`fabricated from large high-quality growth-oriented crystals.
`Sapphire has been grown by conventional methods such as
`the Verenuil, Czochralski and floating-zone techniques. But a
`more modern method (Schmid-Viechnicki 1973) yields high-
`quality crystals of up to 30 cm diameter and 15 cm length.
`
`3. The absorption coefficient
`
`Although the optical transmission band of synthetic sapphire
`is well known, it is not possible to assess its suitability
`for laser windows unless its absorption data are accurately
`found. Several investigators have measured the absorption
`coefficient at a few wavelengths in the infra-red (Deutsch
`1975, Harrington et al 1976, Thomos et al 1988). Figure 1
`summarizes some of these results. Using a combination
`of
`lasers and a Fourier-transform spectrometer,
`infra-
`red transmission and absorption coefficients in the range
`2–20 μm have been measured (Thomos et al 1990). Using
`lasers and accurate calorimetry (Innocenzi et al 1990),
`absorption coefficients in the range 10−5–10−1 cm−1 have
`been measured. Also utilizing such calorimetry, absorption
`coefficients (Innocenzi et al 1990) of sapphire grown at
`60◦ orientation and at 0◦ orientation have been measured
`in some visible, UV and VUV ranges. The present paper
`reports an accurate study of the bulk absorption coefficient
`of sapphire in the range 0.4–2 μm. Two methods are
`employed for this study, namely (i) differential dual-beam
`spectrophotometry (Deutsch 1973) and (ii) an intra-cavity
`optical loss measurement technique (Patel and Charan 1975,
`Patel 1977, 1990) using lasers at various wavelengths. The
`second method is more accurate, but it is limited by the
`availability of a laser at the wavelength of measurement.
`
`147
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`

`
`B S Patel and Z H Zaidi
`On the other hand, the first method offers the advantage
`of being useful over various wavelengths, but its lower
`accuracy restricts its utility in the region where the absorption
`coefficient is large enough to be measurable.
`The absorption coefficient envisaged in the region of
`300–400 nm is around 0.005 cm−1 so that dual-beam
`spectrophotometry can be employed. The experimental
`procedure is similar to one reported earlier (Deutsch 1973).
`It basically consists of measuring the transmission of
`thick samples obtained from bulk crystals grown at 60◦
`orientation at various wavelengths. Then, taking samples
`of varying thicknesses, a graph of the relationship between
`the transmission and the sample thickness is plotted. From
`the slope of this graph, the absorption coefficient has been
`calculated. The limitation of this method arises from the
`fact that the accuracy of measurement is to within about 1%
`and hence it has been essential to take samples of thickness
`more than about 5 cm to allow measurement of absorption
`coefficients of about 0.002 cm−1. With this method, the
`absorption coefficient is measured at a few wavelengths in
`the range 330–460 nm. The results are shown in figure 1. For
`wavelengths greater than 460 nm, the results were not reliable
`and the other method had to be used because the absorption
`in the test samples became very low and the inaccuracies of
`measurements became too large.
`The laser intra-cavity loss measurement technique (Patel
`and Charan 1975, Patel 1977, 1990) has the capability of
`measuring absorption coefficients of about 10−4 cm−1 using
`thick samples and hence is ideally suited to measurements
`in the visible and near-IR regions. A 50 mW He–Ne laser is
`used for the experimental work and the procedure is similar to
`one reported earlier (Patel 1977). Thick samples of sapphire
`are used to keep the error of measurements low, to the level
`of 10−4 cm−1. A He–Ne laser is operated at 632.8 nm and
`then at 1.15 μm to obtain the absorption coefficients at these
`wavelengths. The results are shown in figure 1.
`Figure 1 elaborates the absorption coefficient of sapphire
`for the complete range of its optical transmission. In figure 1,
`the refractive index (Maliston 1962) of sapphire for the
`ordinary ray is also plotted, since this information is essential
`for the study of the suitability of sapphire for windows.
`
`3.1. The optical absorption limitation of sapphire for
`laser windows
`Figure 1 can now be employed to obtain the working
`wavelength range for sapphire laser windows.
`For any
`material to be useful as an intra-cavity laser window, it is
`essential that the optical absorption losses due to its inclusion
`do not exceed 0.2% per transit (even for a high-gain laser
`medium). If a typical window has a diameter of up to 2.5 cm,
`its thickness could be 0.5 mm (it will be seen later that such a
`choice of laser window dimensions is quite adequate). Now,
`imposing the condition that a window of 0.5 mm thickness
`should have (at the most) a single-transit bulk-absorption loss
`of 0.2%, one infers that the material must have a maximum
`absorption coefficient of 0.04 cm−1. When this restriction
`is imposed on the sapphire windows, figure 1 reveals that
`the working wavelength range of sapphire for laser windows
`reduces to 330 nm to 4.0 μm only. However, even this
`
`148
`
`reduced range is really significant since several gas lasers
`operate in this region, including the nitrogen laser (337.1 nm),
`chemical oxy-iodine laser (COIL) at 1.315 μm, argon-ion
`laser (448.0 nm), He–Ne laser (632.8 nm and 1.15 μm),
`HF laser (2.7 μm) and DF laser (3.8 μm). The optical
`suitability of sapphire windows for these lasers can, therefore,
`be investigated.
`
`4. The thickness limitation of sapphire laser
`windows
`
`Our data on the absorption coefficient can now be utilized for
`studying the thickness limitation of sapphire laser windows.
`Thin windows are obviously desirable because they offer
`low bulk absorption. However, gas-laser windows generally
`have to withstand a pressure difference across them which
`produces a uniformly distributed load over the window.
`Hence the window could either fracture or introduce an
`optical distortion of the laser beam transmitted through it.
`When the fracture of the window poses a limitation, one
`can utilize the formula of Sparks and Cottis (1973) and choose
`a minimum thickness tf of the window for withstanding a
`pressure P such that
`tf = 0.433D(P S/A)1/2
`(1)
`where A is the modulus of rupture, D the diameter of the
`window and S the safety factor (taken here as 4, which
`would give twice the fracture-limited thickness). Figure 2
`shows a plot of tf against D. The values of A chosen for
`the calculation is 65 000 psi, being the lowest in its spread
`(table 1 indicates values ranging from 65 000 to 100 000 psi),
`and the pressure P is 1 atm.
`The pressure-induced optical distortion of a laser beam
`passing through the window can also cause a limitation on
`its thickness. Utilizing the formula of Sparks and Cottis
`(1973) for such cases, a maximum thickness to, to restrict
`the distortion to within a reasonable limit, is required:
`to = 0.87D[(μ − 1)(P /E)2(1 − σ 2)2(D/λ)]0.2
`(2)
`where E is Young’s modulus, σ Poisson’s ratio, λ the wave
`length of the radiation and μ the refractive index. Being a
`uniaxial crystal, sapphire has one refractive index μe for the
`extraordinary ray and another μo for the ordinary ray (see
`table 1). If the laser window is kept at the Brewster angle,
`then D would refer to the major axis of the ellipse of contact.
`Equation (2) assumes that the window is either rigidly held
`or stuck near the circumference to the laser plasma tube.
`Figure 2 also shows the plot of to against D for a pressure P
`of 1 atm for the sapphire window at various wavelengths of
`interest.
`It is interesting to note that the maximum thickness is
`mostly limited by the optical distortion of the laser beams
`rather than by fracture. This is all the more so for the shorter
`wavelengths. Also, if the window diameter is more than
`4.5 cm, the limitation is always due to optical distortion at
`all the wavelengths considered. This is to be expected of a
`material of high mechanical strength. Generally, windows of
`about 2.5 cm diameter are used in most experimental work;
`hence, for them 0.5 mm thickness is sufficient to withstand
`a pressure difference of 1 atm for all
`the wavelengths
`considered here.
`
`

`
`Sapphire for laser windows
`
`Figure 2. Minimum thicknesses to and tf of sapphire windows limited by optical distortion and fracture respectively plotted against the
`window diameter for various wavelengths. The pressure difference across the windows is 1 atm. The plots corresponding to various
`wavelengths are: A, for 337.1 nm; B, for 448.0 nm; C, for 632.8 nm; D, for 1.15 μm and 1.315 μm; E, for 2.7 μm; and F, for 3.8 μm.
`
`Figure 3. Intra-cavity losses Lp introduced by sapphire Brewster windows as a function of the angle of incidence φ around the Brewster
`angle θ at various wavelengths. Curves corresponding to laser wavelengths are: A, for 337.1 nm; B, for 448.0 nm; C, for 632.8 nm; D, for
`1.15 μm and 1.315 μm; E, for 2.7 μm; and F, for 3.8 μm.
`
`149
`
`

`
`B S Patel and Z H Zaidi
`
`Figure 4. Total intra-cavity optical losses L introduced by sapphire Brewster windows (kept within their Brewsterization tolerances) as a
`function of the skew angle ψ at various wavelengths. Plots corresponding to laser wavelengths are: L1, for 448.0 nm, 632.8 nm, 1.15 μm,
`1.315 μm and 2.7 μm; L2, for 3.8 μm; and L3, for 337.1 nm.
`
`Table 2. Brewsterization tolerance for 0.5 mm thick sapphire
`windows at various laser wavelengths.
`Lp at φ ≈ θ
`Wavelength
`tolerance
`(%)
`337.1 nm
`0.460
`448.0 nm
`0.041
`632.8 nm
`0.023
`1.15 μm
`0.014
`1.315 μm
`0.012
`2.7 μm
`0.021
`3.8 μm
`0.207
`
`Brewsterization
`tolerance range
`60◦15(cid:9) to 61◦30(cid:9)
`59◦48(cid:9) to 61◦12(cid:9)
`59◦42(cid:9) to 61◦12(cid:9)
`59◦42(cid:9) to 61◦18(cid:9)
`59◦42(cid:9) to 61◦18(cid:9)
`58◦54(cid:9) to 60◦32(cid:9)
`58◦24(cid:9) to 60◦12(cid:9)
`
`φ
`(degrees)
`60◦54(cid:9)
`60◦36(cid:9)
`60◦30(cid:9)
`60◦18(cid:9)
`60◦15(cid:9)
`59◦47(cid:9)
`59◦18(cid:9)
`
`5. The optical suitability of sapphire for Brewster
`windows
`
`The use of Brewster windows within a laser in order to
`obtain output in a fixed polarization is desirable. Also they
`eliminate the limitation on the power-handling capacity of the
`otherwise AR-coated flat windows due to damage to such
`coatings. Figure 1 can now be used to study the optical
`suitability of sapphire for Brewster windows.
`The surfaces of the Brewster windows can be made to
`a flatness of about λ/20 finish and then further polished to
`obtain negligible scattering at 632.8 nm. The faces of the
`plate can be made parallel to within a second of arc. The
`final thickness of 2.5 cm diameter windows is now chosen to
`be 0.5 mm, as discussed above. For such Brewster windows,
`the main sources of optical losses are (i) error in setting the
`
`150
`
`incidence angle φ close to the Brewster angle θ, (ii) error
`occurring due to there being a small skew angle ψ between the
`two Brewster plates and (iii) absorption in the bulk material
`(which has an absorption coefficient of α cm−1). The overall
`optical losses L introduced in the laser cavity for a round-trip
`traversal of the beam can be expressed using results obtained
`by Patel and Charan (1975) to within a good approximation
`as
`L ≈ 2αd + 4(Rp cos ψ + Rn sin ψ )φ≈θ
`where Rp and Rn are the reflectivities of the surfaces of the
`window plate for laser oscillations with the electrical vector
`parallel and normal to the plane of incidence respectively and
`d is the actual single-pass distance traversed by the laser beam
`in the window material of thickness t (taken as 0.5 mm). Also
`these can be expressed as follows:
`Rp = tan2(φ − β)/ tan2(φ + β)
`Rn = sin2(φ − β)/ sin2(φ + β)
`d = t sec β
`β = sin−1[(sin φ)/μ]
`μ being the refractive index of the material at a given
`wavelength.
`
`(6)
`
`(7)
`
`(3)
`
`(4)
`
`(5)
`
`

`
`5.1. Reflection and absorption losses
`Reflection and absorption losses in a window kept at an angle
`close to the Brewster angle θ = tan−1 μ, in the absence of
`skew-angle losses, i.e. ψ = 0, are obtained from equation (3)
`as
`Lp ≈ 2αd + 4Rp.
`(8)
`Figure 3 shows a plot of the variation of Lp as a function of the
`angle of incidence φ around the Brewster angle θ for 0.5 mm
`thick sapphire plates at various wavelengths. The graphs
`for 1.15 and 1.315 μm are practically indistinguishable
`and thus the curve D represents the result of both these
`wavelengths.
`This plot provides tolerances in setting
`Brewster angles.
`If an optical loss Lp of 0.025% is acceptable in addition
`to the Lp value at θ, i.e. 2αd, then the tolerance can be
`obtained from the plots. Such Brewsterization tolerances
`and other salient features are given in table 2.
`
`5.2. Optical losses in the presence of skew angles
`Another important aspect which no worker appears to have
`studied so far but that deserves careful attention is the analysis
`of the optical losses in the presence of skew angles between
`two Brewsterized sapphire plates.
`When the sapphire windows are kept within (or up to)
`their Brewsterization tolerances at various wavelengths and a
`small skew angle ψ is introduced, the total intra-cavity losses
`incurred can be computed using equation (3). Figure 4 shows
`the results. The losses increase rapidly due to Rn sin ψ term
`and hence the smallest possible skew angles are desirable.
`However, if a total optical loss L = 1% (i.e. 0.5% per transit)
`could be permitted, the skew angles for 448.0 nm, 632.8 nm,
`1.15 μm, 1.315 μm and 2.7 μm would be 0.5◦, that for
`3.8 μm would be 0.4◦ and that for 337.1 nm would be 0.3◦
`only. In actual lasers, experimentally it is quite easy to adjust
`skew angles to make them smaller than 0.3◦, but, at the same
`time, it is important to know the implications of including
`larger skew angles.
`
`6. Results and discussion
`
`The present study provides a graph of the variation of
`the absorption coefficient of sapphire over its transmission
`range. The choice of the thickness of the window at a given
`wavelength is found to be limited by the optical distortion
`due to the pressure difference across it, rather than by
`fracture. Since thinner sapphire windows could be used, the
`usefulness of sapphire for various laser windows in terms
`
`Sapphire for laser windows
`of Brewsterization and inclusion of skew angles suggests
`that sapphire may even be comparable to conventional fused
`silica windows for 632.8 nm He–Ne lasers. Brewsterization
`tolerances for 0.5 mm thick sapphire windows at various laser
`wavelengths are given in table 2. As expected (Patel and
`Charan 1975), the Brewsterization tolerance increases with
`decreasing refractive index. For other lasers like the HF laser
`(2.7 μm) and DF laser (3.8 μm), sapphire would definitely
`excel over other materials due to its chemical resistance to
`such acid vapours. Also for COIL (1.315 μm), which is
`emerging as a future high-power CW laser for directed energy
`weapons in the next century, sapphire would probably replace
`all the conventional optical materials like fused silica.
`In
`addition, sapphire windows could also be utilized for other
`widely used lasers such as excimer lasers and copper-vapour
`lasers, especially for excimer lasers operating in the UV
`region, because there are hardly any rugged materials of
`reasonable transmission and hence sapphire windows would
`be useful. Below 0.2 μm, however, even thin sapphire
`windows will perhaps not be adequate because the absorption
`coefficient increases (figure 1) drastically below 0.3 μm.
`
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`151