Spectral Sensors by Carl Zeiss

M e c h a n ic a lly R u le d G r a t in g s
H o lo g r a p h ic G r a t in g s
P la n e G r a t in g s
I m a g in g G r a t in g s

株式会社

スペクトラ・コープ〒164‑0011 東京都中野区中央4‑4‑5第一小林ビル
Tel:03‑5328‑2858 Fax:03‑5328‑2859
URL http://www.spectra.co.jp

Mechanically ruled or holographicallyrecorded - our gratings excel in
precision and efficiency

High efficiency and minimum straylight
Customized production
Many years of experience in the development andmanufacture of diffraction gratings make Carl Zeissfor the right address of diffraction optics. The
technical basis of production are holographic exposuresystems and ultra-high-precision ruling engines which
offer multiple possibilities of modification.
The benefits of holographically produced gratings are
their high diffraction efficiency, even with highgroove frequencies, and their very low straylight.
Holographic procedures also allow the generation ofasymmetric and symmetric groove profiles.
Mechanically ruled Zeiss gratings distinguish by aparticularly uniform groove spacing, resulting in low
straylight. This ensures that spectrometers have a highdetection sensitivity even for low-intensity signals andprovide increased measurement accuracy in thevisible and infrared ranges.

Figure 1:
Zeiss precision diffraction gratings are at the heart ofmonochromators, spectrographs, spectrometers,
spectrophotometers, dye lasers and other laser
types. They are most valuable in curricula, metrology
and in solving special problems.

2

In addition to our standard line of gratings, we also
produce customised gratings, allowing us to meetthe critical demands made by quality-conscious users
in spectroscopy and laser technology.
Carl Zeiss produces:.

nMechanically ruled plane gratings for the
spectral range from 0.5 nm to 40 µm.
Groove frequency 4 to 3000 grooves/mm.

n

Holographically produced plane and ·concave gratings for the spectral range from
150 nm to 5 µm. Groove frequency 40 to6400 grooves/mm.

nDiffractive, holographically produced beamshaping optics (HOE) for laser applications
and beam splitter with a fixed and a variable
splitting ratio, resonator gratings andcollimators for laser diodes.

The classical production method

We produce a wide variety of high-quality gratings
with three ruling engines. Its high mechanicalprecision is further increased by several orders ofmagnitude using an interferometric control system.
Echelette gratings with sawtooth groove profiles
The large groove faces of echelette gratings reflect
most of the light in the diffraction direction
determined by the groove spacing, i.e. the grating
constant, a property which is termed the blaze effect.
The wavelength of maximum grating efficiency, theblaze wavelength, is determined by the angle of thegroove faces. This wavelength can be set as requiredin ruled gratings. The high-precision groove profiles ofZeiss gratings ensure a high efficiency over a wide
spectral range.

Figure 2: Mechanical ruling engine with aninterferometric control system.

Resolving power near the theoretical limit
Mechanically ruled gratings- virtually free ofghosts and straylight
Periodic errors cause grating ghosts which may be
due to irregular operation of the ruling engine. The
Zeiss ruling engines are interferometrically controlledwhich reduces the intensity of grating ghosts to anegligible minimum.
Random groove displacement or local deviations fromthe ideal profile shape, i.e. roughness, cause diffuse
straylight. The choice of adequate metals for theruled film, optimum production conditions, precise
adjustment of the ruling process and the quality of
the replication process keep diffuse straylightextremely low.

The theoretical resolving power of a grating isdefined by the product of total number of grooves
and diffraction order. lt is approximately attained if
the deviations of the wavefront diffracted by thegrating are small compared with the wavelength
used. High optical flatness of the blank and adequate
freedom from systematic, periodic and random errorsof the grating constant are required for this purpose.
The surface quality of the substrate of Zeiss echelettegratings and the exceptional accuracy of the rulingengines fulfil these requirements.
Routine interferometric wavefront examinations in thesecond or higher diffraction orders guarantee thegrating quality; the resolving power exceeds generally
80% of the theoretical limit.

3

The holographic production method

The holographic technique is used to produce gratings
by recording the pattern generated by a fine laserinterference field on a photoresist film.
lt is not a new idea to use light itself for the
production of gratings. Michelson publishedsuggestions to this effect as long ago as 1915. The
production of high-grade spectroscopic gratings,
however, has become possible only with theavailability of high-resolution photoresist films andlasers with shortwave emission wavelength.
On this basis, Zeiss developed new production
technologies and the necessary equipment for the
manufacture of holographic gratings which can beused in a wide range of applications. Holographic
gratings have a low level of straylight and are totally
free from ruling errors. Another benefit offered bythese gratings is that special optical imaging
properties can be realised which cannot be achieved
using mechanical means, e.g. the correction ofaberrations in concave gratings.
Thus, holographic gratings considerably extend therange of application of diffraction gratings. Whether aruled or a holographic grating is the best choice for a
specific application depends on the grating properties
required.

More performance at a lower price due toreplication techniques
With a original replication technique developed by
Zeiss, numerous replicas can be made of a mastergrating produced according to one of theaforementioned methods. The replicas have thesame properties and quality as the master grating.
The replicating process allows the batch production ofgratings, offering price advantages to users.
Zeiss subjects all diffraction gratings- mastergratings
and replicas - to a most stringent quality control:nnnSpectral efficiency and straylight aremeasured in a testing spectrometer.
The diffracted wavefront, i. e. the resolving
power, is checked interferometrically.
The groove profile is checked using a lightmicroscope and an Atomic-ForceMicroscope. This is why Zeiss is able toguarantee constant high quality standards.

In addition, Zeiss offers technical consulting to realize
and optimize your application case.

.
Diffraction gratings with symmetric groove
profilesGratings with symmetric groove profiles areholographically recorded, free from ruling errors andshow extremely low straylight. Special opticalproperties can be realised with holographic gratings,
which cannot be achieved mechanically, e.g. theaberration correction of concave gratings.
Furthermore holographic gratings are easy to produce
in cases where mechanical ruling requires more
effort, e.g. if the ruled areas are large and the groove
density is high.

4

Figure 3: Interferometer setup and beam path for
holographic exposure.

Theoretical efficiency almost achieved
High efficiency is also obtainable with symmetricgroove profiles, provided the grating constant d issmall compared with the wavelength lso that thehigher orders of diffraction are cancelled and allenergy is concentrated in the zero and first orders.
The optimum working range of symmetric gratings isthusgivenbyλ/d>= 0.67.
The higher the groove density, the more theefficiency curves of the gratings shift towards shorter
wavelengths.

Specific advantages of gratings with high groove
densities:n

Higher diffraction orders are cancelled. For
this reason, these gratings offer the largest
possible free spectral range. Order filters or
predispersers are generally not necessary.

nSufficient distance between spectral lines andbecause of that precise wavelength selection
due to high dispersion.

n

In autocollimation, the diffraction angle isbetween 20° and 60°. For this reason, theresolving power in the first order is as high as
in the higher orders obtained with echelle
gratings. The repeated order change typicalof echelles is not necessary; in addition,
efficiency is higher over a wide spectral
range.

n

The light throughput of the units in which thegratings are integrated can be considerablyincreased, as much larger slit widths can beused without any loss in resolving power.

Zeiss optimised the groove profile of holographic
gratings so as to virtually suppress even the zerodiffraction order. The extraordinarily high efficiency ofsuch gratings equals that of ruled echelette gratings
or rather by using polarized light the efficieny is quite
better.
Ionbeam etching of holographic gratings
In ionbeam etching, the grating grooves in thephotoresist coating are transferred to the blanksurface. This increases the mechanical and thermal
stability of the master gratings, permitting their use in
UHV spectral units. There are two methods of groove
transfer: the true-to-shape transfer of grooves where
the original shape of the profile is retained, and theshape-modifying transfer of grooves where, especiallyin reactive ionbeam etching, the shape of the profile
is deliberately changed. This makes it possible tomanufacture aberration corrected concave gratings
with good blaze properties for the spectral range
from soft X-radiation to the infrared. Ionetchedlaminar gratings with a rectangular profile show
extremely low straylight, as the low microroughnessof the blank surface is retained on the groove
surfaces during the manufacturing process.

5

Zeiss laser gratings for all types of dye lasers
Holographic gratings are the best wavelength
selectors for dye lasers. Zeiss laser gratings offer thehigh efficiency and high dispersion required forpolarised laser beams and are successfully used in all
types of dye lasers.
CO2 laser gratings
Mechanically ruled gratings are used in lasers, if ahigh efficiency is required or if the grating issubjected to high thermal exposure. Carl Zeiss
produces mechanically ruled gratings on BK7, ZKN 7,
Zerodur, silicon, copper,molybdenum and steel
blanks for different laser powers. The absolute
efficiency attained for CO2 laser gratings, forexample, is higher than 97%.

Moderately priced gratings for curricula anddemonstrations
Transmission and reflection gratings are available
with and without blaze effect.
They are supplied in slide format
including a coverglass and invarious other versions.

Figure 4:
Efficiency test station for UV. Vacuum chamber,monochromator and light source.
Diffracted wavefront highly flat
Zeiss uses interference fields of high homogeneity torecord holographic gratings on blanks whose flatness isalways better than λ/4. This flatness is fully retained
during the entire production process due to a special
coating process.
Interferometric testing ensures that wavefront
aberrations are always below λ/4, resulting in
maximum resolving power of the gratings. Each andevery one of which is checked if necessary.

Holographic gratings -minimum straylight andfreedom of ghosts
Due to the production technique used, holographic
gratings show neither periodic nor random rulingerrors. Zeiss holographic gratings are free of ghosts
and their straylight intensity lies clearly below that of
the best mechanically ruled gratings.

6

Concave gratings

Gratings with imaging properties for modern
dispersive systems
Zeiss produces concave gratings holographically. The
radii of curvature available are finely graded within awide range. Relative apertures of as wide as 1:1 are
obtainable. Concave gratings combine the dispersive
properties of plane gratings with the imaging
properties of concave mirrors. The spectrum is
produced without auxiliary optical means. Aberrations,especially astigmatism, occur as in image formation
with concave mirrors. Because of an aspheric surface
production of its own, Zeiss is able to offer gratings
also on toric, elliptic or other aspheric surfaces whichshow fewer aberrations than conventional concave
gratings.

nMonochromator arrangements consisting
only of a fixed entrance and exit slit and a
grating rotating around its axis.

nSpectrometers of the polychromator type
without movable parts, with a fixed grating
and a flat diode array covering all
wavelengths of interest.
Spectrographs with a fixed entrance slit,
fixed grating and e.g.a camera withconsiderably reduced astigmatic
deformation of the spectral lines

nn

Even in classical arrangements, e.g. withentrance and exit slits on a Rowland circle,
aberrationcorrected gratings can be used tobest advantage.

Aberrationcorrected concave gratings withconsiderably reduced aberrations
Your benefit from these advantages
Aberration-corrected concave gratings are recorded in
an interferometer set-up adapted to the potential useof the grating. As with holograms, this ensuresaberrationfree images at the operating wavelength
required. Astigmatism is completely compensated forspecific wavelengths and also considerably reduced
over a wide wavelength range. This permits the use ofhigh-aperture gratings with large diffraction angles,
resulting in particularly effective dispersive systems.

Cost advantages due to simple spectrometer
designn With corrected concave gratings it is incontrast to the classical Rowland gratings
possible to vary the position of the focal
curves in a wide range. From it novel simple
versions of the optical design of dispersive
systems can follow.

n

The aberration correction of holographic
concave gratings guarantees high resolving
power and small bandwidths even at highaperturesn

Dispersive systems of high light- gathering
power and compactness can be
implemented using gratings with wide
apertures. The light-gathering power isfurther increased by the high efficiency of
the Zeiss holographic gratingsnStraylight is considerably reduced by simple
design using fewer scattering surfaces. Less
astigmatism allows the use of smaller slit
diameter with reduced transmission ofstraylight. Zeiss holographic gratings are
virtually straylight-freen

Cost advantages due to the elimination ofcollimating and other imaging components
and the use of a simplified mechanicalsystem including easy adjustment

7

Imaging Gratings

Figure 5:

Holographically corrected concave gratings
Concave diffraction gratings combine both dispersive
and imaging properties, making them ideal for use inmodern spectroscopic systems. The number of opticalcomponents of these systems is low: this reduces
straylight and increases the light throughput on the
one hand, and on the other, it also guarantees clear
and compact design and simplified operation. The
basic form of all imaging gratings is the Rowland
circle grating. Its application-specific feature: theentrance slit, the grating and the detector must be
positioned on the diffraction circle of the Rowland
grating, the Rowland circle. The Rowland circle
configuration allows coma-free imaging and ensureshigh spectral resolution despite astigmatic elongationof the slit image. Due to its curved focal plane, theRowland circle configuration is, however, not suitable

8

for use in monochromators or flat-field
spectrometers. Holographically corrected exposureof concave gratings can both optimize the focal
plane and minimize aberrations, such as
astigmatism, spherical aberration and coma, over awide spectral range. Unlike Rowland circle gratings,
these aberrationcorrected gratings exhibit grooves
with a variable spacing and variable curvatures. This
holographic design allows optimum adaptation of a
grating’s imaging properties to the specificrequirements of the spectral unit in which the grating
will be used .This ensures that gratings are obtained
whose imaging properties effectively concentrate theavailable light energy on the detector over a wide
spectral range. Holographically corrected gratings
are ideal for imaging the spectrum on a diode arrayor a CCD sensor. lt is therefore possible to produce
modern, compact high-resolution spectrometers andspectrometer modules at justifiable cost.

Applications

Imaging gratings for monochromators
In monochromators with their fixed entrance slit,
grating vertex and exit slit, the focus of conventional
gratings moves out of the exit slit plane withincreasing spectrum length. Holographic correction
can largely suppress this aberration and markedly
improve resolution.

The simultaneous suppression of astigmatism andcoma in the range specified is a criterion for thequality of holographic design.

LBs
240

220

200

α
∆

LA

β

Entrance
slit

180

LBm

LB

160

Exit slit
140
600

λ (nm)

Figure 6:

Figure 7:

Imaging diffraction grating in monochromator setup

Focal curves (blue) of a corrected monochromator gratingin comparison the uncorrected focal curves (red).

Specifications of standard gratings
Groove frequency

150 - 1900 L/mm

Spectral range

190 - 2500 nm

Focal lengths

50 - 300 mm

Relative apertur eI/D

≥1,6

Types of grating

Sine, echelett- and laminar

Monochromatic straylight≤ 2 x 10 -5 degree -2 ( depending on wavelength)9Applications

Imaging gratings for diode array spectrometers
In diode array spectrometers, the entrance slit, thediffraction grating and the diode array are fixed. For
this reason, they require diffraction gratings which
image the entrance slit on the plane diode array. Thisrequirement cannot be met by classical Rowland
circle gratings. Unlike these gratings, however,holographically corrected gratings with unequal
groove spacing can be designed to obtain an almost
straight focal curve. In addition, astigmatism andcoma can also be corrected over a wide spectral
range.

LA

Entrance slit

−α
β

∆
LB
Diode array

Abbildung 8:
Imaging diffraction grating in spectrometer setup

Specifications of standard gratings

10

Groove frequency

100 - 2600 L/mm

Spectral range

190 - 900 nm

Focal lengths

20 - 200 mm

Relative aperture f/D≥1,6

Types of grating

Sine, echelette and laminar

Monochromatic straylight≤ 2 x 10 -5 degree -2 ( depending on wavelength)Holographically corrected gratings.

The production of holographically corrected gratingsrequires the use of:n sophisticated grating design program formatching the imaging properties to theapplication intended and distributing thediffraction efficiency across the spectrum

optical components for the holography set-up
which meet the critical demands made on
wavefront accuracy and cleanlinessnholography set-ups with a very high stability
and alignment accuracynhigh-precision machining of the grating blanksnreproducible photoresist techniques for groove
generation with nanometer accuracynUnlike the conventional exposure procedure using
plane waves, two spherical waves, partly with specificaspheric deformation, are made to interfere in thephotoresist layer on the blank to generate
holographically corrected gratings. For this, twointerferometer set-ups are primarily used
(Figs 9 and 10).
The principle illustrated in Fig. 9 is employed to
produce gratings with a symmetrical groove profile(sinusoidal and laminar gratings). These gratings offer
a high level of efficiency in the spectral dispersion oflight over a wide range of wavelengths.

r
se
La

n

lnterferometric set-ups

Dreplication technique for the precise andeconomical manufacturing of gratings in
production quantities
Cnvacuum deposition technology to obtain
guaranteemaximum reflection and a low levelof straylight of the gratingsnsophisticated testing technology which ensuresthat optimum properties are obtained during
the production process.

Grating

Figure 9:
Generation of concave gratings with symmetricgroove profile
C and D are the sources of spherical waves

11Holographically corrected gratings

The principle illustrated in Fig.10 is taken as a basis
to produce gratings with sawtooth-shaped groove
profile (often been termed echelettes or blazed
gratings) having specially high efficiency in a specificwavelength range.
In exposure, the two interfering waves strike thephotosensitive resist from two opposite directions
resulting in a steep inclination of the nodal andantinodal planes of the interference field. During thedevelopment proce, the removal of the material
continues along the antinodal planes, with the nodal
planes as barriers. After a specific development time,
the groove profile assumes the desired sawtooth
shape.
To prevent grating ghosts and straylight, anantireflecting coating is vacuum-deposited on thegrating blank back being passed through theconvergent exposure wave.

Ionbeam etching
To improve the optical properties of blazed gratings,
Carl Zeiss also uses ion-beam etching technology:

.

Laser

.

GratingC
D

Figure 10:
Generation of concave gratings with sawtooth-shaped
groove profile
C and D are the sources of spherical waves

12n

Gas ions which are accelerated in an electricfield strike the grating surface after
collimation.

nThey etch away the surface until the grooves
have been transferred into the surface of theblank.

n

lt is also possible to deliberately change thegroove profile, for example, to shift the ·blaze wavelength of blazed holographic
gratings to the spectral range required (e. g.
from VUV to IR).

Sinusoidal gratings

Sinusoidal gratings have a symmetric groove profile.
Gratings with groove frequencies and groove depths
for use over wide range- from -VUV to IR - can beproduced by selecting suitable process parameters,such as the angle between the interfering waves,and the exposure and development times.The groove
depth and number of grooves determine theefficiency for the spectral range in which the grating
can be used. (Definition grating constant d: distance
between the grooves).
As in plane sinusoidal gratings, high efficiency in theregion λ >=0.7xd can be attained in imaging
sinusoidal gratings for light polarised normal to thegroove direction. (Fig.11)

100

⊥

η [%]

80

60

U

40

II
20
0
200 300 400 500 600 700
λ [nm]

Fig.12 compares the spectral efficiency of
unpolatized light diffracted by gratings with different
groove profile.
.

Figure 11:
Spectral efficiency curves of a sinusoidal grating with
1,830 grooves/mm, measurednear autocollimation (∆ =3°)for unpolarized light (U) and light polarized ⊥ and // tothe groove direction.

Figure 12:
Computed spectral curves of the relative efficiency η fordifferent diffraction orders k in the scalar domain(λ≤ 0,6d).
The low efficiency in the high diffraction orders provided
by the laminar gratings make this type of grating ideal for
applications in which the superposition of the spectrumby higher diffraction orders would be annoying.

Sinusoidal

BlazedLaminar
100

3

2

k=1

80

40

60

30

40

k=1

80
k=1

20

η [%]

4

η [%]

η [%]

100

60
40

2
10

20

0

0
200
A

400

600 800
λ [nm]

1000

1200

200
B

20

3

3

4
400

5

0
600 800
λ [nm]

1000

1200

200
C

400

600 800
λ [nm]

1000

1200

13

Laminar gratings

A variant of the sinusoidal profile is the laminar
profile. Its rectangular shape is described by thegroove depth and the land-to-groove ratio. The
advantage of this type of grating over sinusoidal andechelette (blazed) gratings mainly consists in its
ability to suppress higher diffraction orders whose
efficiency can be influenced and minimised byvarying the land-to-groove ratio . In gratingproduction, the developing process is continued until
the photoresist has been removed as far as theblank’s surface and glass strips are uncovered
corresponding to the land-to-groove ratio. In a
subsequent ionbeam etching process, the strips
exposed are etched off until the groove depth
required has been obtained. The photoresist strips
masking the lands during this process are thenremoved from the grating surface.
Laminar gratings produce a markedly lower level ofstraylight than comparable sinusoidal and blazed
gratings: on the one hand, the top surface of thegroove profile retains the high quality of the blank’s
polished surface; on the other, the ionbeam process
is conducted in such a way that there is no
noticeable increase in microroughness, even whenlarge etching depths are required, as in the case of IR
gratings.

.
.

Photoresist
NG 5 glass

Figure 13:
Generation of a rectangular profile for laminar gratings
by means of ionbeam etchinga) photoresist coating on blankb) holographic exposure and photoresist developmentof grooves with a semi-sinusoidal profilec) ion-beam etchingd) removal of remaining photoresiste) application of a reflection coating

14

Replication.
The work and time involved in the production ofholographic master gratings are considerable. A
sophisticated replication technique is used to allow
the production of attractively priced gratings.
nn

The holographically produced grating
patterns are first replicated on so-called
daughter replicas.
These are used to make a large number of„granddaughters“ (Fig. 1); the groove profileof these replicas corresponds to that of themaster.

The reproducibility of accuracies in the nanometer
range allows the production of replica gratings, theoptical properties of which equal those of the mastergrating

Master grating

Daughter
Replication layer

MasterDaughter

Daughter
Replication layer
Figure 14:
Schematic illustration of a two-stage replication
process: from the concave master via a convex
daughter replica to the concave granddaughter
grating (final replica)

Granddaughter
AL- reflection coating

Granddaughter(Replica grating)15

Carl Zeiss
Geschäftsbereich
OEM – Spektralsensorik

D - 07740 Jena
Tel. ++49-3641-642838
Fax:++49-3641-642485
e-mail:info.spektralsensorik@zeiss.de

Internet: http://www.zeiss.de/spectral