Choosing the condenser

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Operation with the spectral devices often needs to apply auxiliary optical units for the optimization of the optical layout: collection and focusing of radiation, matching of apertures and etc. Condensers are applied to solve these tasks. Depending on the set task and conditions there are three general cases:

  • Delivery of radiation into the entrance slit of the spectral device in case when the radiation source is the end of optical waveguide. In this case a condenser is necessary for matching the optical waveguide aperture with the spectral device aperture. For this aim we offer a line of condensers for matching the apertures (F-matcher).

 

  • Radiation is delivered to the spectral device directly from the source (a lamp for example). At the same time it is necessary to collect radiation and deliver it into the spectral device without losses. This task is successfully solved by the condensers from the image transfer units: Object – Entrance slit.

 

  • Radiation at the spectral device exit is necessary to be collected and transferred to the photoreceiver without losses. Depending on the type and the size of photoreceiver an appropriate condenser can be chosen from the image transfer units: Exit slit – Photodetector.

 

 

   
The line of condensers for matching of apertures (F-matcher)

Condensers serve for matching the working apertures at an image formation. Our condensers, being used with optical waveguide, change the entrance aperture of the beam which exits the optical waveguide and match this aperture with the entrance aperture of the spectral device.

Application of F-matcher allows:

  • Matching optical waveguide apertures with spectral devices apertures.
  • Providing high quality of the image over the slit height of our spectral devices.
  • Reduction of the stray light.

 

 

   

Description and operation principle   

Radiation exiting the optical waveguide is propagated within the spatial angle defined by the numerical aperture of the fiber. Effective delivery of radiation into the spectral device is difficult without special devices as a part of radiation is lost (efficiency is reduced), and there are much stray light inside the spectrograph or monochromator.

In Fig. 1 and 2 two ways of radiation delivery from the optical waveguide into the spectral device are presented. In Fig.1 a way of delivery of radiation into the entrance slit of a monochromator or spectrograph directly from the optical waveguide is demonstrated. Figure shows all drawbacks of such a technique which can be eliminated with the use of a special device for apertures matching (Fig.2). Due to such a device the exit aperture of the optical waveguide is matched with the entrance aperture of the spectral device. At the same time there is an opportunity to reduce the stray light inside the device and to use effectively all the radiation exiting the optical waveguide.

In case when the light from the optical waveguide is delivered directly into the spectral device, a part of energy is lost if the exiting angle of radiation from the optical waveguide exceeds the entrance angle of radiation to the monochromator, and\or if the fiber size exceeds the slit size. These losses can be estimated by the loss index (Li):

where
(F1/#) -  is an optical waveguide focal number,

(F2/#)- is a spectral device focal number,

V is a vignetting index which equals to 1 only in that case when the optical waveguide size is smaller than the sizes of the spectral device entrance slit; in other case V < 1.

Thus the higher is the loss index the less energy is lost at the radiation delivery. With application of our condenser matching the optical waveguide exit aperture with the spectral device entrance aperture the ratio and the loss index (Li) depends only on vignetting index V. It should be noted that as the condenser possesses a β – fold linear magnification, so the size of optical waveguide image at the entrance slit of the spectral device is correspondingly β – fold magnified. Vignetting is absent (V = 1), only when the size of optical waveguide image is smaller than the size of the spectral device entrance slit. Condenser is also can be applied at the exit of the spectral device for delivering the radiation to the optical waveguide. In this case the size of the exit slit image is β – times reduced.

 

 

 

Fig.1 Delivery of radiation into the spectral device directly from the optical waveguide.

Fig.1 Delivery of radiation into the spectral device directly from the optical waveguide.

 Fig.2 Delivery of radiation from the optical waveguide into the spectral device with the apertures matching.

Fig.2 Delivery of radiation from the optical waveguide into the spectral device with the apertures matching.

In case when the optical waveguide is a set of fibers (for example, 5 fibers, diameter of each 200 μm, distance 400 μm from each other – see Fig.3), than the entrance slit of the spectral device will obtain the image of the waveguide end. The quality of this image directly depends on the quality of imaging system – condenser.

Replaceable diaphragms in the construction of condensers F-matcher allow the maximal reduction of the stray light in the spectral device. Also there is an opportunity to install a 6- or 8-position filter wheel with replaceable light filters. All condensers of F-matcher line can be supplied with holders for the optical waveguide. These holders allow precise positioning of the waveguide end along three mutually orthogonal directions, including the direction along the optical axis.

Back parts of the condensers F-matcher can be changed according to the desirable spectral range.

Table 1 represents basic characteristics of the line of condensers F-matcher. Among them there are universal utilities for apertures matching as well as instruments developed for special working conditions.

Our specialists will consult you on any questions and choose for you a condenser from our F-matcher line according to the tasks and working conditions you have or they will offer you a new construction optimized specially according to your demands.

 

 

Fig.3 а). Simple double-lens condenser application

Fig.3 а). Simple double-lens condenser application

Fig.3 б). Condenser F-matcher application

Fig.3 б). Condenser F-matcher application

For different spectral devices and optical waveguides we offer several models of condensers F-matcher.

 

Image transfer units: Object – Entrance slit.

Often radiation is delivered to the entrance slit of the spectral device directly from the source (a mercury lamp for example). Two variants are possible:

  • A radiation source (a lamp) is in a considerable distance from the entrance slit of the spectral device. In this case for more effective operation and to avoid radiation losses corrected for infinity condensers for radiation delivery are applied. This type of condensers is schematically shown in Fig.4.

 

  • A radiation source is in a minor distance from the entrance slit of the spectral device. It is reasonable to use condensers corrected for a finite distance. It allows avoiding losses when radiation is delivered to the spectral device. General description of condensers of this type is shown in Fig.5.

 

 

 

 

 

Fig.4

Fig.4

Fig.5

Fig.5

Image transfer units: Exit slit – Photodetector.

Radiation at the spectral device exit is often necessary to be collected and transferred to the photodetector without losses. Depending on operating conditions (the size of photodetector, working spectral range, the height of the exit slit of the spectral device) an appropriate condenser can be chosen.