**6. Holographic optical elements**

PTR glass is a bulk material that is characterized by high homogeneity. Therefore, it is possible to manufacture gratings with high efficient thickness (say, about 1 mm and more). The use of high thickness of the material opens up a possibility to manufacture spatial and spectral filters with outstanding parameters. As known, the selectivity of the Bragg grating depends on its thickness; therefore, it is possible to create gratings with sub nm spectral selectivity and with angular selectivity of <1 angular min.

#### **6.1. Super narrow-band filters for laser diodes and their temperature stabilization of radiation**

The widespread use of semiconductor lasers is stimulated by a number of their advantages [33–35] such as the high efficiency (75–80%), small sizes, simplicity of operation, and relatively low cost. An important advantage of semiconductor lasers is also the possibility of fabricating emitters operating at different wavelengths in the visible, near-infrared, and mid-infrared spectral ranges. Apart from the above beneficial features, the semiconductor lasers have certain drawbacks: their emission is quasi-monochromatic and spectrally unstable. This is caused by a number of factors. The broadening of the lasing spectrum under an increase in the injection current stems from the fundamental aspects of charge-carrier transport and captures into the quantum-confined active region. The lasing spectrum is also affected by the multimode design of the laser cavity. A shift of the spectrum occurs as a result of heating the active region with an increase in the injection current, which causes a reduction in the band gap and, thus, the shift of the lasing spectrum to the longer wavelengths.

does not exceed 1 × 10−4, though this value differs from that obtained with absolute measure-

PTR glass is a bulk material that is characterized by high homogeneity. Therefore, it is possible to manufacture gratings with high efficient thickness (say, about 1 mm and more). The use of high thickness of the material opens up a possibility to manufacture spatial and spectral filters with outstanding parameters. As known, the selectivity of the Bragg grating depends on its thickness; therefore, it is possible to create gratings with sub nm spectral selectivity and with

**6.1. Super narrow-band filters for laser diodes and their temperature stabilization of**

The widespread use of semiconductor lasers is stimulated by a number of their advantages [33–35] such as the high efficiency (75–80%), small sizes, simplicity of operation, and relatively low cost. An important advantage of semiconductor lasers is also the possibility of fabricating emitters operating at different wavelengths in the visible, near-infrared, and mid-infrared spectral ranges. Apart from the above beneficial features, the semiconductor lasers have certain drawbacks: their emission is quasi-monochromatic and spectrally unstable. This is caused by a number of factors. The broadening of the lasing spectrum under an increase in the injection

**Figure 20.** Typical dependence of the RIMA on exposure for bromide PTR glass.

**6. Holographic optical elements**

angular selectivity of <1 angular min.

**radiation**

ments, ~8 × 10−4.

452 Holographic Materials and Optical Systems

This problem can be solved by means of VBG recorded in photo-thermo-refractive glass. Due to high spectral selectivity of recorded holograms, the implementation of such grating inside the external cavity of laser diode can significantly narrow the output spectra. This idea was used widely and proved its advantages. External cavity design based on the VBG can vary (see **Figure 21**), as well as both reflecting or transmitting Bragg grating can be used [26].

The simplest implementation of VBG as an external cavity element is shown in **Figure 21(a)**, where a radiation after being passed through the collimating lens falls normally on the VBG element. Unfortunately, due to a high divergence along the fast axis of the LD output radiation, it is impossible to create the reliable external cavity of LD without additional collimation optics. **Figure 21(b)** shows typical design of external cavity using transmission Bragg grating. The grating works backward and forward, and its diffraction efficiency has to be lower than 80% to couple output radiation efficiently. So there is a need for an additional mirror in the cavity setup to reduce the power loss through the nondiffracted radiation on backward cavity trip. The position of mirrors can be changed, but the number of output channels will remain the same. Also, the cavity designs for coupling the higher-order modes of the LD are also possible. Such designs require the high diffraction efficiency of the grating to provide the maximum output performance and are suitable for wide stripe emitting diodes. Stabilized by means of VBG, the laser diodes show a stable output in the temperature range from 15°C to 75°C [36].

**Figure 21.** Examples of design of external cavity of a diode laser based on VBGs (a) is an example of reflecting VBG implementation and (b) is an example of cavity based on transmitting VBG.

Recent studies of VBG-based external cavity LD show that the implementation of the grating inside cavity significantly increases its selective properties. For example, a grating used in our experiment [37] was recorded with estimated spectral selectivity as great as ~2 nm. We used a cavity shown in **Figure 21(b)** with the transmitting VBG. The emission spectra from such cavity show us two longitudinal modes with the separation of 100 pm and bandwidth of 4–8 pm (**Figure 22**).

**Figure 22.** Emission spectra of laser diode source. Left: spectra recorded with (1) and without (2) grating. Right: the detailed view of the emission line [36].

Similar to the conventional ways of LD stabilization such as Littrow scheme and Litman-Metcalf configurations, using the standard diffraction gratings based on the VBG in the external cavities can provide tuning of the output emission of the source. Merely by the rotation of the grating, we can achieve a tunability along all the gain spectra of the semiconductor crystal that can be really huge, up to 60 nm. An example of such tuning is shown in **Figure 23**.

**Figure 23.** Emission spectra of the external cavity laser diode with different angles of VBG.

#### **6.2. Laser beam combiners**

experiment [37] was recorded with estimated spectral selectivity as great as ~2 nm. We used a cavity shown in **Figure 21(b)** with the transmitting VBG. The emission spectra from such cavity show us two longitudinal modes with the separation of 100 pm and bandwidth of 4–8 pm

**Figure 22.** Emission spectra of laser diode source. Left: spectra recorded with (1) and without (2) grating. Right: the

Similar to the conventional ways of LD stabilization such as Littrow scheme and Litman-Metcalf configurations, using the standard diffraction gratings based on the VBG in the external cavities can provide tuning of the output emission of the source. Merely by the rotation of the grating, we can achieve a tunability along all the gain spectra of the semiconductor crystal that can be really huge, up to 60 nm. An example of such tuning is shown in **Figure 23**.

**Figure 23.** Emission spectra of the external cavity laser diode with different angles of VBG.

(**Figure 22**).

454 Holographic Materials and Optical Systems

detailed view of the emission line [36].

The diffraction efficiency directly depends on the thickness and RIMA, and as shown, this glass has quite a big inflicted refractive index change. Therefore, it is also possible to record multiple gratings inside the single volume of the glass (**Figure 24**).

**Figure 24.** Example of multiple gratings recorded in the single volume of the glass for spectral beam combining.

There is much interest in the use of spectral beam combining (SBC) to combine multiple highpower laser beams into a single high-power one with a narrow spectral linewidth and good beam quality [38]. This idea can be implemented by using several volumes of Bragg gratings for each channel multiplexed in the single volume of PTR glass. Recently, a two and four channel combiner based on the multiplexed reflective Bragg gratings was reported [39]. This approach allows one to develop a combining system with low complexity and better robustness.

#### **6.3. Collimator sights**

The holographic collimator sights are the development of the classical collimator sights. This new kind of design provides the greater transparency of the working aperture compared with the classical collimator and greater parallax suppression. This kind of sights has an open design, which means that the sight can be aimed with both eyes. So a shooter can use the peripheral vision and engage more effectively. Also, due to the properties of a hologram, such sight is very resistant to various injuries and pollution. The hologram is recorded over the entire area of the aperture, which is why the sight remains in the working condition even after a partial pollution and/or damage. Also, one of the main advantages over the conventional sights is the absence of a flare toward a target, which is crucial in a combat.

Basic elements of a holographic sight are shown in **Figure 25**. The operation principle of holographic collimator sights can be briefly described as follows. A radiation from the light source falls on the recorded hologram that creates an image of the recorded mark in the image plane. The high transparency of PTR glass in the visible range (above 90% without AR coating) opens up this field of applications.

**Figure 25.** Holographic sight (a) is the basic scheme of the sight and (b) is the observable image of a mark recorded on PTR glass.

**Figure 26.** Laser action of Nd3+ heavily doped (NNd = 2.5 × 1020) PTR glass measured for mirrors with the reflection of 1% (red curve) and 5% (black curve) [34].

The application of PTR glass can solve the problem of image stabilization, which is necessary due to the instability of laser diode source used in such sights. To date, this problem is solved by adding, into the optical scheme, the achromatizing diffraction elements such as thin gratings, complex two-cavity mirrors, or compound objectives. The wavelength shift caused by laser diode temperature changes can be nullified by spectral selectivity of thick hologram recorded on PTR glass. Because the diffraction efficiency of holograms on PTR glass can reach values of ~95%, an intensity required for the mark observation is rather low. In **Figure 25**, the observable image of holographic mark recorded on PTR glass is demonstrated.

#### **6.4. Distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers**

The concept of DFB lasers was originally demonstrated in 1971 [40] when the laser output from a gelatin film on a glass substrate was obtained for the first time. Two years later [41], a generation from a similar structure on GaAs at nitrogen temperatures was demonstrated. Benefits of such a laser design are pretty obvious: Bragg grating acts as a selective mirror with very narrow reflection bandwidth and, thus, provides a narrow spectral emission output. Since then, DFB lasers had a lot of development, but yet there were no results in creating DFB solidstate lasers.

Doping PTR glass with rare earth elements provides an access to the construction of DFB and DBR lasers because such medium possesses both the laser and holography properties. Recently, first results on laser action on PTR glass were obtained [42, 43]. Later, a generation on heavily Nd-doped PTR glass was obtained in ITMO University [44]. Laser performance is shown in **Figure 26**. Our calculation shows that PTR glass itself is characterized, due to its outstanding homogeneity, by relatively low round trip loss estimated to be ~0.26%, which is comparable to that of commercially fabricated Nd:YAG crystals.

Further investigations of DFB/DBR effect on PTR glass showed that recording a grating inside PTR glass does not affect its lasing properties. For instance, the laser action in the DFB/DBR configuration on Nd- and Yb-doped PTR glasses was demonstrated [45]. In these experiments, the output radiation from both setups (DFB/DBR) and on both types of PTR glasses (Nd and Yb) was obtained. The emission spectra observed show a narrow line with 30 pm bandwidth.
