We report on the development of a tunable Raman fiber ring laser especially designed for the investigation of the transition of molecular oxygen. Singlet oxygen (1Δg) is a reactive species of importance in the fields of biology, photochemistry, and phototherapy. Tunability of the Raman fiber ring laser is achieved without the use of an intracavity tunable bandpass filter and the laser thus achieves a slope efficiency only obtained up to now in Perot-Fabry cavities. A measurement of the action spectrum of a singlet oxygen trap is made in air-saturated ethanol and acetone to demonstrate the practical application of the tunable Raman fiber ring laser for the investigation of the transition of molecular oxygen.
© 2010 OSA
Compared with traditional solid-state lasers, fiber lasers nowadays available present many significant advantages in terms of compactness, reliability and flexibility. In particular continuous-wave pumped all-fiber Raman lasers are versatile light sources which are virtually able to deliver high-power radiation at any wavelength across the 1 – 2.1 μm spectral region [1, 2, 3]. Raman fiber lasers (RFLs) have also been shown to exhibit many attractive capabilities such as multiwavelength operation, pulsed emission or tunability [4, 5, 6]. Nowadays RFLs have many applications in various fields such as fiber sensing, fiber telecommunications, material processing but also in surgery and in the treatment of some oncological diseases [7, 8].
Tunable RFLs have been initially developed with the first objective to provide tunable Raman gain in telecommunication fiber amplifiers  but they have also been used for fiber device testing and for fiber sensor systems. With the recent development of frequency-doubled RFLs generating visible light, it has been additionally argued that a combination of frequency doubling and tuning in RFLs may provide new sources replacing dye or Ti:Sa lasers [10, 11].
In this paper, we report on the development of a tunable RFL especially designed for the investigation of the transition of molecular oxygen. The singlet 1Δg-state (1O2), commonly called singlet oxygen, is the first electronic excited state of the oxygen molecule. It is a metastable state in which molecular oxygen is highly reactive. The singlet state 1O2 of molecular oxygen has been extensively studied for its chemical affinity with biomolecules in photodynamic therapy of cancers [12, 13] and for chemical synthesis .
The transition of molecular oxygen is forbidden at electric dipolar approximation. In practice, this means that both absorption and emission probabilities are very low for the isolated molecule (gas phase). Nevertheless, interactions in dense phase of molecular oxygen with surrounding molecules of inorganic solvents may enhance the radiative process [15, 16, 17] whose efficiency may be increased by three orders of magnitude compared with gas phase.
The mechanisms through which solvents modify the properties of molecular oxygen’s transition are not fully understood. Although several models have been developed to investigate this question, experimental data are difficult to obtain because of (i) weakness of absorption and emission of the considered transition, (ii) low efficiency of detectors in this spectral region  and (iii) poor availability of high power reliable laser sources around 1270 nm.
In this context, Yusupov et al. have recently demonstrated a RFL delivering a power of 5.5 Watt at a wavelength of 1262 nm. From direct optical excitation of oxygen, they have successfully employed this source for the treatment of oncological diseases . Although oxygen-dependent effects are presumably coupled to thermal effects in the experiments of Ref. , the results obtained by Yusupov et al. open promising perspectives for the use of RFLs in the field of cancer therapy. From a more fundamental point of view, Krasnovsky et al. have demonstrated that it is possible to obtain the spectrum of the transition together with an estimation of molar absorption coefficients in various air saturated organic solvents [18, 19, 20, 21, 22]. With the goal of mimicking the mechanisms of the biological effects arising from laser irradiation, they have used the photooxygenation reaction of 1O2 with a specific trap, 1, 3–diphenylisobenzofuran (DPIBF), which leads to colorless endoperoxydes. The variation of absorbance of the trap around 410 nm provides the photooxygenation rate which is linked to the absorption coefficient of molecular oxygen.
A high power narrow-linewidth laser tunable around 1270 nm is of great interest for the investigation of direct creation of singlet oxygen both for medical applications and for more fundamental studies of the transition of oxygen molecules in various solvents. From the latter point of view, such a laser would permit to measure the action spectra of the 1O2 traps around 1270 nm with a good resolution. Up to now, most of the measurements of photooxygenation rates of DPIBF have been performed by using either diode lasers or a wavelength-tunable forsterite laser delivering 200 – 250 ns pulses [20, 21, 22]. The wavelength of the diode lasers used cannot be tuned and the peak of their output power spectrum did not necessarily coincide with the maximum of the photooxygenation action spectrum. Moreover their output power did not exceed 700 mW. Although the forsterite laser previously used can be tuned between 1200 nm and 1290 nm, its linewidth of 3 nm was relatively high and its mean power cannot be increased above 120 mW .
In this paper we demonstrate a high-power tunable RFL operating in a simple geometry and constructed only from commercially-available fiber components. The RFL linewidth is around 1 nm and its wavelength can be continuously tuned from 1240 nm to 1289 nm to precisely measure the action spectra of 1O2 traps. The RFL delivers a total Stokes power of ∼ 2.5 Watt at the maximum available pump power of ∼ 7 Watt. Its output power is almost constant at tuning in the 1240 – 1289 nm tuning range. The configuration of the laser cavity is presented in Sec. 2 and it is compared with other configurations previously reported. Laser performances in terms of output power and tunability are described in Sec. 3. In Sec. 4, a measurement of the action spectrum of a singlet oxygen trap is made in air-saturated ethanol and acetone to demonstrate the practical application of the tunable laser for the investigation of the transition of molecular oxygen.
2. Laser design
Tunability is a property of RFLs which has been investigated from the early developments of these lasers in the 1970s [23, 24]. From this date, several techniques to tune RFLs have been proposed. Various cavity geometries have been studied and several types of fibers have been made to increase tunability ranges and output powers. A recent review about the history of development of tunable RFLs can be found in Section 1 of Ref. . Here we will only provide a brief summary of the principles of operation and of the performances currently reached by tunable RFLs operating both in Fabry-Perot and in ring geometries.
RFLs oscillating inside Perot-Fabry cavities can be tuned over several tens of nanometers by using two main techniques. The beam bending technique consists in gluing the fiber Bragg grating (FBG) mirrors of the laser cavity on a plexiglas beam which may be bent thus providing a mechanical tuning of the RFL . Another technique consists in applying a purely axial compression on the FBGs which are embedded in a highly deformable polymer. Using a tunable Ytterbium-doped fiber laser and a phosphosilicate fiber as Raman active medium, a RFL tunable in the range 1258 – 1300 nm has been demonstrated with the beam bending technique . Using a 1064-nm Ytterbium-doped fiber laser and a germanosilicate fiber as Raman active medium, a RFL tunable in the range 1075 – 1135 nm has been demonstrated by using the axial compression technique of FBGs . Both RFLs are able to deliver Stokes output powers of a few Watt with slope efficiencies around 70%.
In ring cavities, the RFL tunability is commonly achieved from the insertion of a tunable optical bandpass filter inside the laser cavity. The tuning ranges of tunable Raman fiber ring lasers (TRFRLs) which have been demonstrated so far can be wider than tuning ranges of RFLs oscillating in Perot-Fabry cavities. In particular a tuning range of ∼ 100 nm (from 1495 nm to 1600 nm) has been recently reported in a Raman laser made with a tellurite fiber . A tuning range in excess of 65 nm from 1486 to 1551 nm has been demonstrated in a TRFRL made with a germanosilicate fiber . However the germanosilicate and the tellurite TRFRLs have maximum Stokes output powers of 420 mW and ∼ 500 mW, respectively. TRFRLs demonstrated up to now have typical efficiencies around ∼ 25% and their output powers is lower than tunable RFLs constructed from Perot-Fabry cavities. Note that this point is not necessarily detrimental for telecommunication applications and that several low-power multiwavelength TRFRLs have been designed for wavelength-division-multiplexing systems [26, 27].
Wavelength dense multiplexers (WDMs), circulators or fiber couplers are often used to build ring lasers. As these fiber components do not have pronounced wavelength-dependent losses, ring geometries easily favor multiwavelength Stokes emission . Tunable bandpass filters are therefore used to introduce wavelength-dependent losses inside the laser cavity thus providing single-wavelength emission with a tunability which is often limited by the performances of the optical filter itself. Here we propose a simple ring configuration in which the only narrow-bandwidth of the Raman gain curve of a phosphosilicate fiber (see Fig. 1 of Ref. ) is used to restrict the RFL Stokes emission to a single wavelength. With this configuration the use of an intracavity bandpass filter is not required and the RFL is simply tuned by tuning the wavelength of the pump laser.
Our TRFRL is schematically shown in Fig. 1. The pump laser is a commercially-available Ytterbium-doped fiber laser (Manlight ML10-CW-R-OEM-TUNE-1080) which is tunable between 1060 nm and 1100 nm. It delivers a randomly-polarized output beam with a maximum output power of 7 Watt. The pump light is launched inside the fiber cavity from port 1 to port 3 of a 1080 nm /1280 nm WDM coupler. As in Ref. [3, 11], we have used a phosphosilicate fiber having a large P2O5 shift of ∼ 40 THz as compared to the Stokes shift of ∼ 13.3 THz in germanosilicate fibers. With this phosphosilicate fiber the Stokes wavelength is shifted from only one Stokes cascade in the wavelength region of interest for the investigation of the formation of singlet oxygen (i.e., around 1280 nm).
The TFRFL oscillates in a ring cavity which is formed by recoupling the output end of the 500–m long phosphosilicate fiber at port 4 of the WDM. With the ring cavity arranged in this way, cavity losses vary according to a sine function of wavelength which is maximum around 1080 nm and minimum around 1280 nm. As the P2O5-related gain peak of the phosphosilicate fiber is also around 1280 nm, one could first imagine that Stokes emission will simply build up only in this wavelength region. However a SiO2-related Raman gain bandwidth grows around 1350 nm concomitantly with the emergence of the 1280 nm Stokes component. As the cavity losses are relatively weak around 1350 nm, the RFL can easily deliver two Stokes lines and the radiation around 1350 nm can become much more intense than the radiation around 1280 nm at high pump power. To eliminate this dual-wavelength operation, we have chosen to increase cavity losses in order to push the power threshold of the Stokes component at 1350 nm above the maximum pump power available (i.e., 7 Watt in our setup). As shown in Fig. 1, this has been achieved from the insertion of a 80/20 (manufacturer data) fiber coupler inside the laser cavity. With this intracavity coupler, the power threshold for Stokes emission around 1280 nm is increased around 4 Watt but the fraction of Stokes power extracted from the laser cavity is as high as ∼ 80%, which gives a high slope efficiency.
3. Laser performances
In the TRFRL presented in Fig. 1, a forward-propagating pump wave and a forward-propagating Stokes wave copropagate in the counterclockwise direction whereas only a backward-propagating Stokes wave circulates in the clockwise direction. The power carried by the backward-propagating Stokes wave has been simply measured by using a power-meter. Measuring the spectral power density of forward-propagating light over a wavelength span ranging from 1050 nm to 1300 nm by using an optical spectrum analyzer (OSA), we have computed the ratio between the powers carried by the pump wave and by the forward-propagating Stokes wave. From an additional measurement of the total power carried by forward-propagating light, we have simply obtained the power carried by the forward-propagating Stokes wave and by the pump wave.
Figure 2(a) shows the power characteristics of our TRFRL measured for a pump wavelength of 1085 nm which produces Stokes emission around ∼1268 nm. The total Stokes power plotted in Fig. 2(a) is the sum of powers carried by the forward- and the backward-propagating Stokes waves which are extracted from the cavity at the output fiber coupler (see Fig. 1). The laser power threshold is around ∼3.7 Watt and the Stokes power delivered at the maximum available pump power of 7 Watt is around ∼ 2.5 Watt. The corresponding slope efficiency of ∼ 75% is close to the efficiency of tunable RFLs oscillating in Perot-Fabry cavities [6, 11].
In the TRFRL shown in Fig. 1, the ratio between the powers carried by the forward- and backward-propagating Stokes waves can be adjusted by using the two fiber polarization controllers (PCs) schematically represented in Fig. 1. Adjusting the PCs, we have found that it is possible to reduce the power of backward-propagating Stokes wave down to a negligible level thus maximizing the power of the forward-propagating Stokes wave. However this kind of operating regime does not correspond to a situation in which the total Stokes power is maximized. We have found that the total Stokes power is maximized when the Stokes powers carried by forward- and backward-propagating Stokes waves are roughly comparable. The power characteristics presented in Fig. 2(a) have been obtained by incrementing the incident pump power by steps of ∼0.3 Watt and by slightly readjusting the PCs at each step in order to get the maximum Stokes power. Let us emphasize that the TRFL is placed into a environment-isolated box and as far as the pump power and the PCs are fixed, the fluctuations of the powers carried by forward-and backward-propagating Stokes waves do not exceed a few percent over several hours.
Figure 2(b) shows the optical power spectra of the forward-propagating Stokes wave. These spectra have been recorded at three different pumping levels and for a pump wavelength of 1087 nm. The full-width at half-maximum (FWHM) of the Stokes spectra approximately increases from ∼ 0.5 nm to ∼ 1 nm when the pump power is increased from 4.9 W to 7 W. The TFRFL output power spectrum presents a bell-shaped profile which does not vary much with the incident pump power. This feature is very different from the one found in RFLs oscillating in Perot-Fabry cavities which have spectra taking a double-peak structure at high incident pump power .
Figure 3(a) shows that the wavelength of Stokes emission linearly varies with the wavelength of the tunable pump laser. Our TRFRL laser can be tuned over 49 nm from 1240 nm to 1289 nm by tuning the pump laser between 1065 nm and 1100 nm. The tuning range is not limited by any bandpass filter but only by the tuning range of the pump laser. As shown Fig. 3(b), approximately 1.6 Watt of total Stokes power is delivered over the whole tuning range at an incident pump power of ∼ 5.3 Watt. This power is more than three times greater than the maximum power previously measured in TRFRLs made with intracavity bandpass filters [9, 25]. In our setup the Stokes power could be increased to ∼ 2.5 Watt over the whole tuning range but a significant amount of Stokes light may then be fed back towards the pump laser. To avoid a subsequent damage of the Ytterbium fiber laser, the level of optical feedback towards the pump source has been continually monitored. Using the maximum pump power of 7 Watt, we have been able to tune our TRFRL from ∼ 1250 nm to ∼ 1280 nm at its maximum output Stokes power of ∼ 2.5 Watt without the risk of damaging the pump source. The tuning range at high power will be extended in the near future by inserting a fiber isolator between the Ytterbium fiber laser and the TRFRL.
4. Measurement of the action spectrum of DPIBF in air-saturated ethanol and acetone
In this section, our TRFRL is used to measure the absorption spectrum of the transition of molecular oxygen into two distinct solvents. As shown by Krasnovsky et al., measurement of photoxygenation rate of chemical traps can be used for this purpose [18, 19, 20, 21, 22]. Krasnovsky et al. have demonstrated that 1270 nm irradiation causes degradation of DPIBF dissolved in air saturated solutions. This degradation is due to directly-excited singlet oxygen (1Δg) which reacts with DPIBF leading to colorless endoperoxydes. The decrease of the concentration of DPIBF is monitored through a decrease in the absorbance of the solution at a wavelength of 410 nm which coincides with an absorption band of DPIBF. The 1O2 production rate is directly proportional to the concentration of DPFIB which is known from the variation of absorbance at 410 nm according to Beer-Lambert’s law.
Figure 4 shows the normalized action spectra of DPIBF photooxygenation in air-saturated ethanol and acetone upon irradiation of the TRFRL. The spectra are normalized to the maximum photoxoygenation rate for each solvent. In these experiments, the concentration of DPIBF is ∼ 50 μmol.L−1 both in ethanol and acetone. The solutions which are placed into a cubic quartz cell of 1 cm long and 1 cm large are illuminated around 1270 nm over a duration of 10 min. The volume of the solutions is 1.4 mL. The solutions are shaken during all the irradiation to prevent any sedimentation of the trap. The diameter of the infrared beam irradiating the quartz cell is ∼ 1.6 mm and its power was ∼ 0.35 W.
The variation of absorption of DPIBF is monitored by a 405 nm laser beam, simply by measuring the laser power transmitted before and after irradiation. The diameter of the blue laser beam is 8 mm and its power was ∼2 μW. The relation between the measured laser power and the trap concentration reads21] and L = 1 cm, the photooxygenation rate can be easily obtained from
Our results confirm that the maximum of absorption the spectrum of the molecular oxygen band is located at 1273±1 nm, as in previous studies by Krasnovsky et al. . The width of the action spectra is ∼ 15 nm FWHM both for acetone and ethanol. It is comparable with values previously reported by Losev . Furthermore the relative photooxygenation rates in acetone and ethanol coincide with the values reported by Kransovsky et al. .
In this paper we have demonstrated a high-power RFL which can be continuously tuned between 1240 nm and 1289 nm. The laser design is simple and the cavity is constructed only from commercially-available fiber components. Contrary to ring lasers previously demonstrated, the laser wavelength is not tuned from an intracavity tunable bandpass filter but by tuning the wavelength of the pump laser, the narrow bandwidth of the phosphosilicate gain curve being exploited to restrict the laser emission to a single wavelength component. The RFL tuning range is thus only limited by the tuning range of the pump laser. The TRFRL presents a slope efficiency of ∼ 75% which is comparable to the slope efficiency of RFLs oscillating in Perot-Fabry cavities. It delivers a total Stokes power of ∼ 2.5 Watt at the maximum available pump power of ∼ 7 Watt.
We estimate that the TRFRL demonstrated in the present paper can be of a specific interest for the investigation of direct creation of singlet oxygen both for medical applications and for more fundamental studies of the transition of oxygen molecules in various solvents. In water, some questions concerning the determination of the 1,3-diphenylbenzofuran photooxidation rate remain open . In this solvent, the quantity of dissolved oxygen is about ten times less than in acetone and ethanol. We have demonstrated that the TRFRL can be used for the reliable measurement of action spectrum of DPIBF in air-saturated ethanol and acetone with only 300 mW of 1270 nm light. With a maximum output power greater than 2 Watt, the TRFRL presented here could thus overcome the weak absorption of molecular oxygen in water. Moreover the designed TRFRL is only limited in terms of accordability and delivered output power by the pump laser. The tunable TRFRL thus opens the possibility to investigate in photochemistry the excitation band of singlet oxygen in solution phases where shifts in the absorption can be important .
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