Wideband fiber amplified spontaneous emission (ASE) sources are needed for the manufacturing and characterization of fiber components, optical metrology, fiber sensors, spectroscopy, and medical imaging, including optical coherence tomography. They are expected to provide spectral power density of several orders of magnitude higher than halogen bulb sources coupled into a single-mode fiber. Commercially available sources usually have a bandwidth of lower than 50 nm around 1050 nm, 50–100 nm around 1570 nm, and 100 nm around 1900 nm. Wideband sources based on supercontinuum generation can cover a much wider region. Wang presented a supercontinuum fiber source covering the spectrum from 1800 nm up to 2700 nm [1]. The commercially available supercontinuum source from NKT Photonics covers a spectral band of 450–2400 nm. While supercontinuum wideband sources have numerous attractive features, their cost makes them prohibitive for many applications.

We will concentrate on ASE sources for 2000 nm band in this Letter. Attractiveness of the ASE sources is based on their low cost and their capability to provide a stable unpolarized wideband signal with a white rf-spectrum. The ASE sources for 2000 nm band are mostly based on thulium- and holmium-doped fibers. The ASE was observed in a thulium-doped fiber for the first time to our knowledge by Oh et al. [2]. The silica fiber with a multicomponent glass core was core-pumped with a titan–sapphire laser, and ASE with a bandwidth of 77 nm centered around 1900 nm was observed. It was recognized that thulium-doped fibers have the broadest emission bandwidth of all rare-earth doped fibers [3]. Wide gain bandwidth was used for wideband amplification [4,5] and widely tunable lasers [6]. Halder et al. reported on a bismuth–thulium-doped fiber ASE generator covering a spectral range of 1800–2050 nm [7].

Holmium codoping of thulium-doped fibers is used to extend the emission to longer wavelengths. The Tm:Ho-doped double-clad ZBLAN fiber pumped at a wavelength of 803 nm was used to generate ASE with a total spectral width of 380 nm peaked at a wavelength of 2065 nm and a power of about 2 mW [8]. Bandwidth narrowing was observed for higher pump powers. Dorosz achieved an ASE bandwidth of 63 nm in thulium–holmium-doped fibers [9]. The emission of thulium-doped fibers can be also reduced to a shorter wavelength using the erbium as the codopant. Luminescence emission with a bandwidth of 420 nm was observed in antimony-germanate glasses codoped with ${\mathrm{Er}}^{3+}/{\mathrm{Tm}}^{3+}$ ions [10].

ASE sources based on large-mode area of double-clad fibers were proven to be suitable as high-power wideband sources. High-power and wideband sources are generally required in fiber optic gyroscopes, optical coherence tomography, etc. Tm-doped silica fiber cladding-pumped by a diode laser at 803 nm was reported to generate 88 mW with a spectral bandwidth of up to 42 nm centered at 2005 nm [8]. A short piece of the highly thulium-doped tungsten tellurite double-clad fiber pumped by a 792 nm laser diode was used to generate 34 mW ASE with a bandwidth up to 140 nm around 2000 nm [11]. Halder et al. presented the ASE source based on the double-clad ytterbium-sensitized thulium-doped fiber with an integrated power of 1 W and a 10 dB bandwidth of 330 nm covering the spectrum from 1770 to 2100 nm [12]. Large-mode-area double-clad fiber was used to generate 8 mW of power over a bandwidth of 280 nm that was narrowed to 36 nm when the output power achieved 11 W [13]. The fiber was pumped by beam-shaped 790 nm diode bars from both ends.

In this Letter, we describe a low-cost solution made from a core-pumped thulium–holmium-doped fiber and a pair of wideband fused fiber couplers that spectrally combine forward and backward ASE. Extremely wideband operation is achieved in this way covering more than 645 nm at a level of $-10\mathrm{dB}$. Figure 1 illustrates the experimental setup of the ASE source. The ASE source includes a thulium–holmium-doped fiber as an active medium. The fiber was fabricated in our laboratories from a preform prepared by modified chemical vapor deposition (MCVD) and solution doping as described elsewhere [14]. The fiber has a numerical aperture of 0.15 and a core diameter of 8 μm. The alumina-rich silica core is doped with 1800 ppm of thulium and 360 ppm of holmium. The spectral absorption of the fiber is shown in Fig. 2. The thulium absorption peak achieves $56\mathrm{dB}/\mathrm{m}$ at a wavelength of 1642 nm. The active fiber was pumped by an amplified CW laser working in a wavelength range of 1571–1611 nm. This corresponds to a left wing of absorption peak corresponding to the ${}^3{H}_6\to {}^3{F}_4$ transition of thulium ions. The ${}^3{F}_4+{}^5{I}_8\to {}^3{H}_6+{}^5{I}_7$ energy transfer process between the thulium and holmium ions then populates the excited state of ${\mathrm{Ho}}^{3+}$ [9]. The pump wavelength should be selected with respect to a trade-off between the thulium absorption and an efficiency of the booster amplifier that is based on erbium-doped fiber.

 

Fig. 1. Experimental scheme of the ASE source. DFBL—distributed feedback laser, BA—L-band booster amplifier, THDF—thulium–holmium-doped fiber, C1,2—couplers, MP—measurement point (cf. text).

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Fig. 2. Spectral absorption of the thulium–holmium doped fiber used in the experiments.

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We used 320 cm of the active fiber in the ASE source as a compromise between the requirements to completely absorb the strong pump and to achieve maximum magnitude of the forward ASE. We needed fiber sufficiently long enough to absorb almost all the pump and prevent damage to the sensitive detector of the spectrometer. On the other hand, fiber that is too long reabsorbs ASE at short wavelengths while ASE is not augmented anymore at long wavelengths. We combined the backward and forward ASEs in order to build an ultrawideband ASE source as illustrated in Fig 1. The backward ASE is much stronger than the forward ASE. The forward ASE is pronouncedly shifted toward the longer wavelengths as a result of reabsorption of the ASE along the fiber [4]. We tried standard communication coarse wavelength-division multiplexers to couple the pump into the active fiber and to get the backward ASE out. Unfortunately, these multiplexers have a lot of spectral transmission dips above 2000 nm. We finally sacrificed some of the pump power and used a wideband fused coupler instead. The coupling strength of fused biconical taper couplers depends on the overlap of modes of tapered fibers and therefore it is wavelength dependent [15]. We selected the coupler with a 50% coupling ratio at 1600 nm. The source was pumped at a wavelength of 1611 nm with a power of 480 mW. The spectral transmission of the coupler for backward ASE decreases toward the longer wavelengths and compensates the positive slope of backward ASE until its peak is achieved. After this point a dip in the output spectrum appears that is ultimately compensated by forward ASE. This allows us to achieve relatively flat ASE spectrum at the output of the source. Both the couplers used in the source were selected from standard commercial products designed for telecommunication applications based on their measured spectral transmission functions (Fig. 3). The solid blue curve shows the transmission in the backward ASE branch (ports $1\to 3$), while the solid red curve shows the transmission in the pump branch (ports $2\to 1$). ASE in various points of the setup can be seen in Fig. 4. The blue curve shows the forward ASE transmitted over the output coupler. It was measured with fibers disconnected in the measurement point MP (Fig. 1). The dip at 1950 nm caused by the absorption of the holmium ions is clearly visible and can be attributed to transition ${}^5{I}_8\to {}^5{I}_7$. The emission extends far beyond 2300 nm. Such emission cannot be observed in fibers doped solely with thulium. The green curve shows the backward ASE transmitted over the first coupler that was measured in measurement point MP. The pump peak visible at 1611 nm results from a reflection at the unused port of the first coupler that was equipped with an angle polished connector (APC). While the reflection is greatly suppressed by the APC connector, it was still necessary to make several tight loops at this idle pigtail to further suppress the backreflection. Finally, combined forward and backward ASE were measured for the closed backward ASE path, as represented by the magenta curve in Fig. 4. As can be seen, the backward ASE is further attenuated by the second coupler. The forward ASE branch of the second coupler should extend to the furthest wavelengths possible. Its transmission function is shown by the dashed lines in Fig. 3. In this way a relatively flat output spectrum can be achieved. In Fig. 5, the output spectrum is labeled with the achieved bandwidths to demonstrate the efficiency of this method. Looking at the 3 dB bandwidth it should be kept in mind that the ripple is much larger than 3 dB. Bandwidths achieved 645 nm at the level of $-10\mathrm{dB}$ and 807 nm at $-20\mathrm{dB}$.

 

Fig. 3. Transmission functions of coupler C1 (solid) and C2 (dashed). Blue shows the transmission functions for backward ASE path.

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Fig. 4. Backward ASE (green), forward ASE (blue), and combined ASE (magenta) measured for the spectrally flat ASE source.

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Fig. 5. Combined spectrum of the ASE source.

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While the flat spectrum is good for comparing diverse ASE sources, it does not meet the requirements of spectroscopy. The sensitivity of detectors usually grows superlinearly towards the longer wavelengths and then drops suddenly when the energy of photons becomes lower than the bandgap of the material of the active area. A practical ASE source for spectroscopy should exhibit stronger power spectral density at short wavelengths where the sensitivity of detectors is small. To enhance the ASE at short wavelengths we simply changed the ports of the first coupler. Now the backward ASE peak is delivered with minimum loss toward the output coupler, where it is combined with the forward ASE. We also changed the pump wavelength to 1571 nm where the first coupler is able to couple more power into the active fiber as can be seen from Fig. 3. Moreover, our L-band amplifier used for amplification of the pump has better efficiency at this wavelength delivering 640 mW of power into the ASE source. The forward, backward and combined ASE in various points of the setup can be seen in Fig. 6. As a figure of merit we have chosen the ratio of the measured spectral density of ASE generator to the measured spectral density of a halogen source coupled into a single-mode fiber (SMF28). This enhancement factor is shown in Fig. 7. The new configuration denoted as a source for spectroscopy offers an enhancement factor up to 25 dB compared to 15 dB for the spectrally flat ASE source. The ASE source overcomes the halogen bulb source for wavelengths longer than 1540 nm. It should be noted that the efficiency of the fiber ASE source is smaller than the efficiency of the ASE sources based on fibers doped with thulium only. The backward ASE peak around 1800 nm is approximately two times smaller when compared to fibers with similar concentrations of thulium ions under the same experimental conditions. The decrease of thulium luminescence related to introduction of holmium ions was observed also by Dorosz et al. [9]. It is caused by a desired energy transfer process between the ${\mathrm{Tm}}^{3+}$ and ${\mathrm{Ho}}^{3+}$ ions as well as by an unwanted upconversion process when an excited ${\mathrm{Tm}}^{3+}$ ion transfers energy to already excited ${\mathrm{Ho}}^{3+}$ ions, ${}^3{F}_4+{}^5{I}_7\to {}^3{H}_6+{}^5{I}_5$. Other loss mechanisms are also under investigation.

 

Fig. 6. Backward ASE (green), forward ASE (blue), and combined ASE (magenta) measured for the ASE source optimized for spectroscopy.

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Fig. 7. Enhancement factor with respect to a SMF28 fiber coupled halogen source for the spectrally flat ASE source (blue) and for the source optimized for spectroscopy (magenta).

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In conclusion, we have developed the spectrally flat ASE source with $-10\mathrm{dB}$ bandwidth of 645 nm with an integrated power of approximately 1.3 μW. We believe that it is the widest bandwidth achieved in rare-earth-doped fiber based ASE sources to date. To achieve such a bandwidth, we combined the forward and backward ASEs generated in a thulium–holmium-doped optical fiber using wideband fused fiber couplers. We also optimized this source for spectroscopy. It has an integrated power of 9 μW and covers a spectral band of more than 800 nm.